Almost certainly, the firing pins are sticking in the primers. Long Hunter sells some pins that eliminate this problem.
However, my experience is that lubricating the pins with graphite, rather than petroleum oil, will prevent them from sticking for a while.
You can remove the firing pin shields with a spanner (spring clip) wrench. When you get it apart, clean everything with alcohol and lubricate with dry graphite. It wouldn't hurt to polish the tips of the pins while you've got it apart.