what operating system are you on?
You can try this Control panel(classic View) > Power options > Put it on balanced.
try that if that doesnt work then i dont know.
Sounds like you have a process that is not allowing the computer to go to sleep. Have you checked to see if your anti virus is running a scan when you try to put it to sleep? Your best bet is to check out all of your running processes and then you will probably be able to determine what is running and rewaking/not allowing the computer to go to sleep.