Macbook Pro won't sleep when lid is closed

I have a relatively new Macbook Pro.
When I close the lid, it won't sleep... It just stays on, and if I put it in my case it gets too hot.

Any way to fix this?

Try doing a PRAM reset. Shut the machine down. Turn it back on and immediately press and hold Command + Option + P + R. Continue holding until the system chimes 3 times. Then release. See if that does the trick.

do you use any apps that prevent the computer from going to sleep? InsomniaX for example

No InsomniaX or other similar programs... Just the regular stuff. I'll try the reset suggestion and update with the results.

The reset seems to have worked, but time will tell. Last night I manually put the computer to sleep (with a fully charged battery.) This morning the battery was dead...

Thanks for the help!