What do you think went wrong?

I had a compresor go bad, put in a new one, recharged, held charge for a month, then blew hot air, got charged again, blew ICE cold air for a few days, then blew hot. So the guess was its leaking from some where, dye went in and it was recharged, ac blew ice cold, then the next day was blowing hot again. When they checked it, they could not find where it was leaking from, wasnt leaking any place past the firewall. Whats the culprit? Evap core? I dont drive the car any more, but eventually I would like to fix the a/c in it because those few days it blew cold air were awesome.

Neil