I have a really weird problem. I made a 3D simulator of an "assembly line" as a part of a college project. Among other things it needs to be able to detect when a box object passes in front of sensor. I tried to solve this by making a model of a laser and checking if the box collides with it. I had some problems with BoundingSpheres of models meshes so I simply create a BoundingSphere and place it in the same place as the model. I organized them into a list of BoundingSpheres called "spheres" and for each model I create one BoundingSphere. All models except the box are static, so the box object has its own BoundingSphere (not a member of the "spheres" list). I also implemented a picking algorithm that I use to start the movement. This is the code that checks for collision:

if (spheres.Count != 0)
        {     

            for (int i = 1; i < spheres.Count; i++)
            {                    
                if (spheres[i].Intersects(PickingRay) != null && Microsoft.Xna.Framework.Input.ButtonState.Pressed == Mouse.GetState().LeftButton)
                {
                    start = true;
                    break;
                }
                if (BoxSphere.Intersects(spheres[i]) && start)
                {                        
                    MoveBox(0, false);//The MoveBox function receives the direction (0) and a bool value that dictates whether the box should move or not (false means stop)
                    start = false;
                    break;
                }
                if (start /*&& Microsoft.Xna.Framework.Input.ButtonState.Pressed == Mouse.GetState().LeftButton*/ && !BoxSphere.Intersects(spheres[i]))
                {
                    MoveBox(0, true);                        
                    break;
                }


            }

The problem is this: When I use the mouse to move the box (the commented part in the third if condition) the collision works fine (I have another part of code that I removed to simplify my question - it calculates the "address" of the box, and by that number I know that the collision is correct). But when I comment it (like in this example) the box just passes trough the lasers and does not detect the collision (the idea is that the box stops at each laser and the user passes it forth by clicking on the appropriate "switch"). Can you see the problem? Please help, and if you need more informations I will try to give them. Thanks