I have configured nagios in my environment and i have used the following setting for a service which monitors the disk space for every 10 min ,

Normal check interval =10 min
retry check interval = 1 min
Maximum attempts =2

But accoring to this values i should recieve my first alert after 10 min and then from there for every 1 min. But in my case i am recieving my first alert and then next alert comes to me after 20 min. Can some one help me in fixing this.