Why won't my Mac sleep/wake when I close/open it?

Did it always act this way? What OS are you running, The last two releases have had a lot of sleep/wake issues.

If you have not changed the OS you could have a corrupt .plist file.

Create a new user account and use it just as a test. Does it act the same or as you expected. If the behavior stops you have eliminated a hardware problem. You can go back to your main account and try to track it down (deleting a corrupt .plist file is a "fix" for strange application behavior).

If this answer is acceptable please remember to return and mark it accepted.