Compressor Problem -- Regulator?

Hello Folks. I have a Porter Cable 4.3 gallon Air Compressor. It pumps up to 150PSI, and it's real sexy.

Recently, I had it looked over by a friend's dad. Ever since, it won't properly power my guns. It still charges up to 150PSI on the tank, and the regulator claims to be presenting 120PSI. The guns, however, act as though they are receiving pressure that is too low for them to operate properly.

What's happening (with two identical Senco Framers (FramePro601) and a Senco Roofer) is one nail will fire and sink 1/2 to 3/4 of the way into pine. If I have the regulator at 120, the piston will usually recoil completely, and the gun will load the next nail. Every 6-7 nails, I have to open the gun, push the piston back, and advance the nail manually.

If I set the regulator around 100, the piston will not recoil, and the nail can not advance.

The guy who worked on my compressor (He did it for free) did some playing with the regulator. It was leaking, and he said he put new thread tape on it. I am hoping he just over-tightened something in there and I can fix it myself without much trouble.

Does anyone have any advice about fiddling with regulators? I've looked around online for a schematic of the compressor -- or even just a generic regulator schematic -- and can't seem to find one.

Thanks in advance.