A/C blows hot air

Recently, the air conditioner stopped blowing in our car so we took it into the shop. They said it was a relay issue, causing the compressor to not kick on. So they replaced a "harness". After getting the car back, It worked for a few hours, but I noticed it was blowing luke warm air every now and then. So I turned it onto the indoor circulation and it was getting cold again. But I also noticed that when turning off the car, often the next time we drove, it was back to luke warm air again. Sometimes messing with the buttons got it working and sometimes messing with it did nothing. So I dropped it back off at the shop, only for them to inform me they left it running and it was blowing cold air. Therefore, it seems to them like nothing is wrong with it. So I need help figuring out what this could be, I am not really car savvy but I do know the general basics(or can learn by other means). So here is what I know/did so far: Car stops blowing cold air(bought a refill of refrigerant), was blowing cold air when filling, as soon as I drove no cold air, took it into the shop (told a electrical relay issue not getting to the compressor) they replaced the harness, When driving it is now randomly blowing luke then cold air. There are no lights indicating that it is turning off and on, it just turns to warm air/cold air at random times. The other day in stop and go traffic it completely stopped working, likewise to the turning the car off.

Ac doesnt blow cold air, everything was fine just one day no cold air, it blows air all the bottens work, but no cold air, what could it be?