Have you reset the SMC, as per > Resetting the System Management Controller (SMC)
1. Shut down the computer.
2. Unplug the computer's power cord.
3. Wait fifteen seconds.
4. Attach the computer's power cord.
5. Wait five seconds, then press the power button to turn on the computer.
Does your Display Sleep using a HotCorner...?
To check, open Mission Control and
1. Click the Hot Corners... button
2. Select "Put Display to Sleep" in a corner
3. Click OK
Then move your pointer to that Corner and check to see if your Display Sleeps.