I recently installed an Apex Tactical polymer forward set sear and trigger kit on my M&P9. This included new trigger, trigger return spring, sear, sear spring, striker block, and striker block string . When I took it to the range yesterday, I had two failures to fire and had light firing pin marks on the primer. I didn't try any more rounds, but retried the same ones and no luck. I didn't tune the trigger bar loop to add overtravel, because I don't mind not having any. Any ideas? Should I tune the trigger bar loop?

put the original striker spring back in , that is what I had to do after my Apex upgrade

I hate to break it to you, but you do need at least some over travel on the trigger. This make sure the striker doesn't drag on the sear and gives you a margin of error to allow for wear/debris. You might want to put some magic marker or Dykem on the striker plunger/block and make sure the striker is running free of interference.