Select "LO" on both temperature settings and disengage the compressor with the "EC" button. After the system stabilizes, you should have the same (warm) temperature air out of both sides. If you do, the problem is in the cryogenic system. If the driver's side is still warmer, the problem is in the heater system.

The cryogenic problem is most likely that you are low on refrigerant, as MB Doc indicates.