What Phil said about running out of air, especially if your compressor is running a lot.
I use a 3M Accuspray gun which comes with a regulator (of sorts - it's just a manual valve with a gauge on it). I set my regulator at the compressor (5HP-60 gal) at 50-60 lb to be sure I have enough pressure at the gun, then set the gauge at the gun at 15-20 psi on the gauge with the trigger pulled and paint discharging.
