Rear A/C Problem

I havea 2002 Chevy venture 3.4L entended LS with rear Air/Heat. We were having problems with all the AC in the vehicle took it in and had the compressor, condensor, drier, seals, and expansion valve replaced. the front works like a champ. The back starts to cool and then gets warm and then hot. I was wondering what else it could be. The front blows cold all the time.

Sounds like I would take it back to the shop and tell them to finish the job....... could be a bad expansion valve in the rear tho....