Try a brand change and you will possibly make your issue go away.
I normally shoot Hull Cartridge Co. I went to Fiocchi because there was a 'bad batch' from Hull in their CompX 21g cartridges with misfires (same in any gun, and about 1 in 25 affected).
The Fiocchi go fine in all but two Merkels (both sidelocks, but different models). Headspace and primer depth both seem fairly typical, and strike looks solid enough - but clearly not enough to do the business.
Now back on Hull, and all is well again.
