If just the tank is losing pressure then it could be one of the tank fittings or it could be the compressor check valve.
To test the check valve, first remove the gray filter from the front of the compressor. Then turn the compressor on, with the compressor running the air filter port should have a good amount of suction (insert your own joke here) If there is air blowing out of that port then you have a bad check valve which is allowing your tank to leak down. If you find this is the problem, let me know and I will ship you out a warranty valve.