When using n60 nailer, one nail goes all the way in, but the next nail only goes halfway in and the some don't go in at all. Don't know what's wrong.

Worn orings in gun and the trigger valve can/will cause problems as you describe. Remove the nails and leave open the latch. Lighty tap the nose of the gun against coiled rubber hose. The driver/piston should stay in the top position (if the driver is down or comes down after test, replace driver oring). Now push the nose against the rubber hose and activate/hold trigger, while holding the trigger/gun in this position, the driver will /should stay down under pressure. At this point no air should leak out of the nose of the gun. If air leaks out of the nose, examine the lower bumper for cracks ( replace as needed ). Finally remove the trigger and examine the firing valve pin. If it has a plastic pin, looks worn and wobbles from side to side, most likely needs to be replaced. If however it has a metal pin and it seems to fit snugly, do not change unless it is leaking air. Good luck