Ignition problem. Please help!!!

I have a 97 Blazer. When I try to turn it off the ignition will not unlock for me to get the key out. I work for a Chevy dealer and they just changed the ignition cylinder today. But after a few short runs, it happened again. Does anyone know what else the problem could be? I've read up on similar problems but there seems to be a lot of different reasons as to why this could be happening. I would appreciate any info anyone can give me.

Piss your employer off and take your truck to another dealer!
Could be the cylinder was not properly installed...there is a small piece of metal that is staked over the tumblers...if it is not staked properly, it will prevent the cylinder from turning.
Also have them check the shift interlock solenoid.