Your gun has the second style ejectors and the ejector mechanism is in the fore-end. The springs are quite strong, and after firing and opening the gun, the springs are compressed making the gun harder to open.

I reload and use Cheddite primers on all of my low-pressure loads and have never had a problem with the firing pins "sticking".

Look at the empty hull and see if there are drag marks on the end to see if the firing pins are dragging when opening. If so, like Dr. Drew stated, try another low pressure brand shell.