The problem may be with worn or torn o-rings in the trigger assembly. Part number 903754 will replace all the o-rings in that assembly plus other o-rings in the gun to get it tuned up and firing like new.
Usually that's due to low air pressure. Increase your pressure a little but make sure you don't go over the recommended pressure- I rarely go over 110 psi for any of my nail guns. Your gun need air to retract as well as to drive.