A/C blows warmer on drivers side than passenger side

I have a 1997 Dodge Grand Caravan. The A/C sometimes works well but most of the time it hardly cools. The compressor, dryer, condenser, etc. have been replaced. I've noticed that the driver's side blows warmer than the passenger side. I'm wondering if the dash control unit could be the problem or if there is something else I need to check.