First I would make sure the firing pin isn't broken.
Are they any modifications made to the gun other than the upper?
Sometimes a lightened hammer can cause light firing pin strikes. If you have a lightened hammer try putting the original back in. Did you change the mainspring? If you have a lightened mainspring try going back to the original.
You could be getting bad ammo.
I used to shoot a ton of standard velocity Remington and stocked up on it when I found it on sale. I found out just how bad their QC had gotten after I had stored it a few years. I consistently run into about a 12-15% failure rate.
Pulling the bullets tend to show either no priming compound, or no powder. Rarely both.
An enlightened zeal for the energy and efficiency of government will be stigmatized as the offspring of a temper fond of despotic power and hostile to the principles of liberty.
Alexander Hamilton, author of Federalist No. 1. 10/27/1787