The OP wasn't specific on when the phone won't go to sleep but I found this thread after I've been experiencing this issue.
I'm currently having an issue with the phone screen staying on when I press the power button to turn the screen on (but keep locked), and after I miss a phone call (screen turns on to display call but doesn't turn off).
I have my timeout setting at 2 minutes and have waited several minutes past this and I had to turn the screen off manually.
Anyone able to replicate this issue?