When you first got it, it worked. __> So it didn't have any manufacturing defects as some described. (No crack in a cup or piston head.)
Try cleaning and oiling first. You have never oiled it and you don't use it much... So I would suspect the seals are dry and sticking... If not too dry, then it may be minor...
Let me explain-- This is something that was drilled into me as an apprentice and I, in turn, drilled into my apprentices. Most of the seals in an air nailer are just rubber o-rings. If you don't keep them oiled... One thing that happens is that they run dry and start dragging, then wearing excessively. With being dry and heat (the OP lives in a normally hot arid area) the seals sometimes get gummy. Other times, if really dry, they will get brittle and split or chip...
Everyday for many years, I started each day with a few drops of pneumatic oil in each gun I used. I usually ran thousand of nails, staples, brads, pins, etc. Most guns, when the seals wore out, I either bought kits or went to a hydraulics supply and bought o-rings. Sometimes you will run enough nails through to need a head, but I doubt the OP has shot that many nails.
- If it were firing, but weakly (not sinking nails all the way home)... then oil or worn seals... just like if the air pressure were not enough.Then you would hear in "how" it fires. More so if you hear a slight hiss as it fires. ...Or not a crisp op when it fires.
But if it sounds as if it is firing crisply, leaving a divot from the head into your work, but not firing a nail, then check the magazine mechanism and/or the fasteners.
"Don't worry, I saw this work in a cartoon once."
"Usually learning skills and tooling involves a progression of logical steps."