I am trying to install 1/2" engineered maple floors over a subfloor consisting of 3/8" plywood with 1"x12" boards beneath. I am using a Norge 18 gauge nailer with 11/4" 18 gauge L Cleats. I've got the nail gun dialed up to 115psi, which is the maximum recommended air pressure for the gun, and the cleats are not going in all the way to the base of the groove so the next boards tongue will fit in. As is, I am stuck with about 1/16" to 1/32" gap between the boards due to the cleats not going down flush.

Do you recommend dialing up the pressure more or going with a shorter cleat if available? It almost seems as if the cleat is going through the hardwood and 3/8" ply but is stopping when it hits the 1"x12" lumber.

If you run the pressure up anymore you will damage the o rings and it will start to leak air and not cycle. Id take it back (im assuming its new ) and exchange it. Those cheap guns work good if you get a good one but they seem to be hit or miss on that.

I'd first try longer cleats. Whats the floor joist spacing? Maybe your getting some bounce.

i install engineered flooring with an angle stapler. A nice one will come with an adjustable boot or wedge kit for different flooring thicknesses. you will save money on fasteners and will experience less fatigue with a trigger fastener than with a standing hammer driven device. they are also very reasonably purchased through amazon.com.

Sounds like you have not adjusted the boot/shoe for the 1/2 flooring.
It may be adjusted for 3/8 or 5/16 flooring.