You are probably getting the same amount of temperature drop at the vents...it'* just that when you're running the car in the morning the cabin air temp is about 10 degrees cooler, so if it starts out 10 degrees cooler it stays 10 degrees cooler.
You could be low on freon..are you R12 or did you convert to R134? If R134 you can get it at a Parts Store. To be sure of what'* going on, you need to put a set of gauges on the system and see what pressures you are running on the high and low side. If you add refrigerant without knowing if you're low you can make it even less efficient.
You could also have a problem with a cooling fan, or a plogged condenser. It could be plogged up externally...lot'* of debris in the fins...or there could be restrictions internally preventing the refrigerant from properly traveling through the condenser and disapating the heat. Try running a stream of water on to the condenser in front of the radiator and see if your system begins to cool better, that could indicate a problem in that area.
You might also have a problem with a air-mix door not closing all the way and letting to much outside air in.