Sounds like a sealed system problem.
This would be an expensive repair as you need a pro. Often it makes more sense to put the dollars towards a new unit.

Check that the compressor is on all the time other than when it goes through a defrost cycle. You can usually hear it but also should be able to feel it vibrating.

Make sure that the compressor has been on for at least an hour and then check your evaporator coils. If there is only frost in one area (usually where the freon enters the coils) or no frost then it is a sealed system problem.

If you do this check shortly after or during a defrost cycle you could end up with the wrong diagnosis.

Check yur warranty as many units have a longer warranty on the sealed ystem.