Long story short, I installed an Apex Comp spring kit on my MP9FS last year, no issues. I recently put a FSS kit in but kept the Comp springs in to see if it would work. All was fine for about 2-300 rounds and Im starting to get a an issue where the trigger wont reset. Happens prolly once every 20 shots or so. Sometimes happens 1 out of 100. Any idea which spring may be causing this? Trying to keep the pull as light as possible.

Thanks

What sear spring are you using?

Off the top of my head it was the one from the Comp spring kit. I installed it about 3 months ago. Problem arrived after I installed the FSS kit about 2-300 rounds deep.

That sounds like the problem. The FSS kit comes with a heavier sear spring to prevent sear flutter/dead trigger.

Thanks ill swap it and give it a try

I'm with check here. There should be a note there about using the 1/8" diameter spring on the FSS sear and ensuring your sear housing block has the wider hole in it.