The first thing that comes to mind is they are not being oiled properly. Follow the directions for the number of drops of air tool oil to put in daily. If they have been run dry, this makes quick work of the Orings and requires rebuilding the nailer with new Orings.

The second thing is if contamination has gotten into the cylinder bottom thaat can happen if a jam has ever occured.

Dis-assemble the nailer and inspect for these conditions.