There are quite a few programs around that prevent sleep. You can try using something like Task Manager or Process Hacker to kill one at a time to find the offending one; rather a slow and boring process sadly. If you have just installed something new and the problem started that is probably the reason. I recently had this with a program called Air Parrot; I was able to pin it down fairly quickly because it was a recent install. I don't know of any quick way to find out what has rejected a Sleep request: maybe there will be an indication in the Windows lots.
