I have a M&P Pro 5" 9mm. It was made in 2010 but it has the large sear. I had no issues with it (granted I did not have hundreds of rounds through it) before I put an Apex FSS kit in it. Now 1 in every 100-150 rounds I have a round that has a light primer hit. If I put the round back in, it fires just fine. I did get some oil in the striker channel, but I had the gunsmith clean it out (I could have done it myself, but it had been a tough week, he said that there was not that much oil in there). It does have the newer style striker assembly. I did not put in any of the springs in that came with the competition spring kit that I bought. I never even touched the striker. I did leave the factory trigger spring in. Before I start mucking with it I wanted to get ideas as to why I am getting a light primer hit every 100-150 rounds.

My thought is that it is because you put in the non-factory parts, and they were not "drop-in." The best procedure is to re-install the factory parts, unless your gunsmith can get it to work.

I started having random light strikes on my 9L Pro Core a few weeks ago. I dropped in my spare striker and no more light strikes. The striker had about 40,000 rounds on it. I measured both the strikers and the difference was only .001", kinda hard to believe that would be enough of a difference but I'm not going to argue with the results. I never did get to try firing the rounds for a second time because they happened during a match and when I cleared the misfire, the rounds went into the mud puddles. That's part of the joy of pistol shooting in the Pacific Northwest.

__________________
Minimize the variables

Bkreutz,
I do have a spare striker assembly and I was thinking of putting that in. It is new from S&W. The guy I bought the Pro from said that he only put about 250 rounds through it. We have now put about 600-700 rounds through it. Who knows what the real round count is..
I hear ya about the mud. Went shooting at the local outdoor range on Thursday. It started to pour and then hail (the hail stones were about an inch across). All the brass we swept up was soaking since it had been sitting in puddles. We had to stick it outside to air it out a couple days later. It now has green spots on it, lol. I just put a load of brass in the tumbler to try to clean it up.

If you've got a spare, throw that in and see if the problem goes away. If not then drop back and punt. (I'm gettin too old to run around in the mud shooting at wilting paper targets , but somehow I keep talking myself into doing it.)

__________________
Minimize the variables