Why does my MBP not always fall asleep?

[oxband]

Sometimes I put the computer to sleep, but when I take it out of my bag later, it's really hot and the battery has gone down so it clearly hasnt gone to sleep. This has happened with both Snow Leopard and Lion. Any ideas what can cause this problem?

 

[GGJstudios]

Sometimes I put the computer to sleep, but when I take it out of my bag later, it's really hot and the battery has gone down so it clearly hasnt gone to sleep. This has happened with both Snow Leopard and Lion. Any ideas what can cause this problem?

You may find some useful ideas here:

Mac OS X: Why your Mac might not sleep or stay in sleep mode
Determine why your Mac wakes up from sleep
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