It could be a number of things.
Suspects include charge level, system refrigerant restriction, weak compressor or clutch, airflow disruption, and the list goes on.
The first check has to be a refrigerant pressure check, both low and high side. After the proper test setup, readings should be taken and the center vent and outside ambient temps have to be noted.
Without proper data, the only thing you'll get will be guesses which may or may not (more likely) be useful.
A quick check that can be made is to hand-test the temperature of the inlet and outlet evaporator lines to see how much temperature differential is present. A vehicle with a correctly charged and operating system will exhibit temps on both sides that are similarly cool, usually in the 35-45 F range. A system that is low on charge or has certain other faults will have an outlet line that is substantially warmer than the inlet.
Steve
