David, get a good A/C tech to check the pressures in the system. Have the center outlet temp checked. There are specs for all of these parameters. This is easy inexpensive stuff to get done. I would never recommend just throwing a little more refrigerant in because it's not quite as cold as it used to be. There are a number of sensors, switches, etc, that can produce that symptom. If you overfill, well you know..
