1995 S10 Air Conditioning

I bought an S10 from a shop a couple of weeks ago, and after two days, the AC quit blowing cold. i took it back, and they replaced the high pressure switch on the back of the compressor. Two days later, it quit blowing cold again. The compressor is engaged, and I assume that the freon level is full since it has been back at the shop twice for this. Any ideas on this?

Thanks in advance!