Are both shell rims cuts the same depth? I had one double that had one chamber rim cut less than the other. One shell dropped further in than the other. Caused firing pin drag when opening. That gun also had seen little use. And it was very picky about brands of primers. Soft ones were a problem more than hard primers.
If both chamber areas are normal then I take a look at the shoulder area of the firing pin for wear or rounding. Is there any wear on one or both firing pins? Or you could either try another brand of shells or if lazy file the firing pin down. But that may just hide the real problem and I hate to alter firing pins or trigger parts unless I have no other choice.
