that action job probably included a reduced power mainspring and maybe cutting the length of the strain screw. A typical gun with stock spring has ballpark about 40% more strike force than it takes to ignite a "typical" round but not all primers are typical. A reduced power mainspring typically will lower the spring force by maybe 30% leaving you with about a 10% margin. If the strain screw has been shortened, you can easily get misfires. Or if you get some ammo with stiff primers, same thing.

Bottom line, you just learned why SW uses the springs they use: to have enough hit to ignite all brands of ammo. You also learned why spring makers put the large warnings about never using reduced power springs in life defense guns.