Some of the quick connects are very poor for air flow, in hand with too small fittings and lines attached to them, a big problem causing not enough air, is leaks in the system, which also can over work the compressor/motor. You might have 120 psi static, but only be a getting lower pressure at useage point, due to small lines, poor connectors, fittings, etc. when in use. Looong lines are a problem too. A pressure gauge at the useage point can be an eye opener.
A funny thing this is the same for air, liquid, and electrical. Is what looks like a regulater on the lower tank bypassed before it goes to the upper tank? Might be some issues there if not.