i am having an issue with my game in XNA. My jump sprite sheet for my character does not trigger when i hit the space bar. I cant seem to find the problem. Please help me. I am also put the code below to make things easier.

namespace WindowsGame4 {

public class Game1 : Microsoft.Xna.Framework.Game
{
    GraphicsDeviceManager graphics;
    SpriteBatch spriteBatch;
    // start of new code
    Texture2D playerWalk; // sprite sheet of walk cycle (14 frames)
    Texture2D idle; // idle animation
    Texture2D jump; // jump animation
    Vector2 playerPos; // to hold x and y position info for the player
    Point frameDimensions; // to hold width and height values for the frames
    int presentFrame; // to record which frame we are on at any given time
    int noOfFrames; // to hold the total number of frames in the spritesheet
    int elapsedTime; // to know how long each frame has been shown
    int frameDuration; // to hold info about how long each frame should be shown
    SpriteEffects flipDirection; // SpriteEffects object
    int speed; //rate of movement
    int upMovement;
    int downMovement;
    int rightMovement;
    int leftMovement;
    int jumpApex;
    string state; //this is going to be "idle","walking" or "jumping".
    KeyboardState previousKeyboardState;
    Vector2 originalPlayerPos;
    Vector2 movementDirection;
    Vector2 movementSpeed;

    public Game1()
    {
        graphics = new GraphicsDeviceManager(this);
        Content.RootDirectory = "Content";
    }

    protected override void Initialize()
    {
        // textures will be defined in the LoadContent() method
        playerPos = new Vector2(0, 200); // starting position for the player is at the left of the screen, and a Y position of 200
        frameDimensions = new Point(55, 65); // each frame in the idle sprite sheet is 55 wide by 65 high
        presentFrame = 0; // start at frame 0
        noOfFrames = 5; // there are 5 frames in the idle cycle
        elapsedTime = 0; // set elapsed time to start at 0
        frameDuration = 80; // 80 milliseconds is how long each frame will show for (the higher the number, the slower the animation)
        flipDirection = SpriteEffects.None; // set the value of flipDirection to none
        speed = 200;
        upMovement = -2;
        downMovement = 2;
        rightMovement = 1;
        leftMovement = -1;
        jumpApex = 100;
        state = "idle";
        previousKeyboardState = Keyboard.GetState();
        originalPlayerPos = Vector2.Zero;
        movementDirection = Vector2.Zero;
        movementSpeed = Vector2.Zero;


        base.Initialize();
    }

    protected override void LoadContent()
    {

        spriteBatch = new SpriteBatch(GraphicsDevice);
        playerWalk = Content.Load<Texture2D>("sprites/walkSmall"); // load the walk cycle spritesheet
        idle = Content.Load<Texture2D>("sprites/idleCycle"); // load the idle cycle sprite sheet
        jump = Content.Load<Texture2D>("sprites/jump"); // load the jump cycle sprite sheet
    }

    protected override void UnloadContent() // we're not using this method at the moment
    {
    }

    protected override void Update(GameTime gameTime) // Update method - used it to call a number of other methods
    {
        if (Keyboard.GetState().IsKeyDown(Keys.Escape))
        {
            this.Exit(); // Exit the game if the Escape key is pressed
        }
        KeyboardState presentKeyboardState = Keyboard.GetState();
        UpdateMovement(presentKeyboardState, gameTime);
        UpdateIdle(presentKeyboardState, gameTime);
        UpdateJump(presentKeyboardState);
        UpdateAnimation(gameTime);
        playerPos += movementDirection * movementSpeed * (float)gameTime.ElapsedGameTime.TotalSeconds;
        previousKeyboardState = presentKeyboardState;
        base.Update(gameTime); 
    }

    private void UpdateAnimation(GameTime gameTime)
    {
        elapsedTime += gameTime.ElapsedGameTime.Milliseconds;
        if (elapsedTime > frameDuration)
        {
            elapsedTime -= frameDuration;
            elapsedTime = elapsedTime - frameDuration;
            presentFrame++;
            if (presentFrame > noOfFrames)
                if (state != "jumping")
                {
                    presentFrame = 0;
                }
                else
                {
                    presentFrame = 8;
                }
        }
    }

    protected void UpdateMovement(KeyboardState presentKeyboardState, GameTime gameTime)
    {
        if (state == "idle")
        {
            movementSpeed = Vector2.Zero;
            movementDirection = Vector2.Zero;
            if (presentKeyboardState.IsKeyDown(Keys.Left))
            {
                state = "walking";
                movementSpeed.X = speed;
                movementDirection.X = leftMovement;
                flipDirection = SpriteEffects.FlipHorizontally;
            }
            if (presentKeyboardState.IsKeyDown(Keys.Right))
            {
                state = "walking";
                movementSpeed.X = speed;
                movementDirection.X = rightMovement;
                flipDirection = SpriteEffects.None;
            }
        }
    }

    private void UpdateIdle(KeyboardState presentKeyboardState, GameTime gameTime)
    {
        if ((presentKeyboardState.IsKeyUp(Keys.Left) && previousKeyboardState.IsKeyDown(Keys.Left)
            || presentKeyboardState.IsKeyUp(Keys.Right) && previousKeyboardState.IsKeyDown(Keys.Right)
            && state != "jumping"))
        {
            state = "idle";
        }

    }

    private void UpdateJump(KeyboardState presentKeyboardState)
    {
        if (state == "walking" || state == "idle")
        {
            if (presentKeyboardState.IsKeyDown(Keys.Space) && !presentKeyboardState.IsKeyDown(Keys.Space))
            {
                presentFrame = 1;
                DoJump();
            }
        }
        if (state == "jumping")
        {
            if (originalPlayerPos.Y - playerPos.Y > jumpApex)
            {
                movementDirection.Y = downMovement;
            }
            if (playerPos.Y > originalPlayerPos.Y)
            {
                playerPos.Y = originalPlayerPos.Y;
                    state = "idle";
                movementDirection = Vector2.Zero;
            }
        }
    }

    private void DoJump()
    {
        if (state != "jumping")
        {
            state = "jumping";
        originalPlayerPos = playerPos;
        movementDirection.Y = upMovement;
        movementSpeed = new Vector2(speed, speed);
        }
    }

    protected override void Draw(GameTime gameTime) // Draw method
    {

        GraphicsDevice.Clear(Color.CornflowerBlue);
        spriteBatch.Begin(); // begin the spritebatch
        if (state == "walking")
        {
            noOfFrames = 14;
            frameDimensions = new Point(55, 65);
            Vector2 playerWalkPos = new Vector2(playerPos.X, playerPos.Y - 28);
            spriteBatch.Draw(playerWalk, playerWalkPos, new Rectangle((presentFrame * frameDimensions.X), 0, frameDimensions.X, frameDimensions.Y), Color.White, 0, Vector2.Zero, 1, flipDirection, 0); 
        }
        if (state == "idle")
        {
            noOfFrames = 5;
            frameDimensions = new Point(55, 65);
            Vector2 idlePos = new Vector2(playerPos.X, playerPos.Y - 28);
            spriteBatch.Draw(idle, idlePos, new Rectangle((presentFrame * frameDimensions.X), 0, frameDimensions.X, frameDimensions.Y), Color.White, 0, Vector2.Zero, 1, flipDirection, 0); 
        }
        if (state == "jumping")
        {
            noOfFrames = 9;
            frameDimensions = new Point(55, 92);
            Vector2 jumpPos = new Vector2(playerPos.X, playerPos.Y - 28);
            spriteBatch.Draw(jump, jumpPos, new Rectangle((presentFrame * frameDimensions.X), 0, frameDimensions.X, frameDimensions.Y), Color.White, 0, Vector2.Zero, 1, flipDirection, 0);            
        }
        spriteBatch.End(); // end the spritebatch commands
        base.Draw(gameTime);
    }
}

}