I don't know what kind of compressor you have or how big it is but I'm going to assume it's a typical one where you have two gauges. One gauge will show the tank pressure and one gauge will show the air regulator pressure (hose). There should be a knob that turns to regulate what you want your regulator pressure to be. Make sure the air drain valve typically found underneath the compressor tank is not leaking air when the tank is full. Remember, when you open a sink valve, it's the same as having an open air hose so it's not uncommon to have the PSI read different. Are you keeping the compressor plugged in and running when using it so it can replenish the air you're using?