This should probably be in Handguns - Autoloaders
It seems a lot of people have issues with hard primers in the S&B ammo. I personally have never had any issues, but have only fired it from a Beretta 92FS which is known to have a lot of firing pin energy, compared to most striker-fired pistols, and subcompact hammer-fired pistols such as your LC9.
That said, a failure to fire every 4 or 5 rounds still seems excessive. I might suggest calling Ruger to either get a stronger hammer spring or have them look at it. Also, try another batch of S&B. It is possible you just got a box with exceptionally hard primers (even by their standards)