I installed the ultimate striker block in my M&P c40 and now I am getting light primer strikes and the striker is not resetting sometimes. Has anyone else has this issue and how did you correct it.  I have also used nothing but factory quality ammo when testing it.

Best to contact them. I had some light strikes and it was found to be a defective striker block. They sent me another one.

I went through that with my 9MM FS first, then when I moved the parts to my .40/.357 SIG FS I had similar issues.

Contacting APEX is probably the way to go.  If they don't get it fixed and you want the pistol bad enough to work on it yourself I can tell you what I found with mine.  I got all kinds of advice, most not accurate/effective, but finally someone on the M&P forum mentioned two things, then explained it to me.  This is the way I understand what he told me.

1.  the striker not resetting sometimes (dead trigger) - it was called "sear flutter".  I replaced the APEX sear spring with a factory spring.  The APEX sear had a rounded off upper/rear contact surface.  I stoned the rear of the sear flat, but kept the same angle as the bottom/rear surface of the sear.

Result - no more dead trigger issues.

2.  light strikes - It was called a "timing issue".  The trigger bar is not completely moving the striker block out of the way of the sear before the sear release the striker.  The striker hits the striker block and finishes moving it out of the way but that slows the striker down to the point that it does not set off the primer.  I used a pair of pliers to twist/tweak the top (dogleg some people call it) rear hump on the trigger bar backwards/towards the rear of the frame just a little bit.  That results in the trigger bar contacting the striker block sooner in the trigger pull and therefore getting the striker block fully out of the way of the striker before the sear releases the striker.

After those two "fixes" I've put 450 rounds through the pistol without a single failure of any kind.  Prior to that it was 2 to 3 rounds per magazine that wouldn't fire for one issue or the other.

It's a pain in the butt when you have multiple causes for a problem.

Oh, my 9MM FS had the sear flutter issues from the factory.  I didn't know what it was, but that's why I bought the APEX CAEK kit in the first place.  Dead trigger, but all I knew was I could pull the trigger and nothing happened.  That happened the day after I bought it.  Shot it, cleaned it, trigger stopped working while dry firing it after the cleaning.

Sear flutter and dead trigger are also much more of an issue on the older style M&P sears that had the 1/16 diameter sear spring (stupid, stupid, stupid idea from the get-go).  The sear housings with the 1/8 spring are much less prone to the dead trigger.  Speed Shooter Specialties sells the sear housing block with the larger spring (when they have them in stock).  You can upgrade your gun yourself and potentially fix any dead trigger issues you have.

Alternatively you can see if S&W will swap out your sear housings but they'll want to do it at the factory I'll bet.

Updates?

How old is your pistol? Is it an older model?

I would be tempted to purchase and install the newest sear housing block assembly, trigger bar, and slide stop. Get a new sear and striker, too. Parts wear out, and S&W has made changes to their design. The current production of M&P pistols are pretty solid. I just purchased a compact and the stock trigger isn't half bad at all, although there is nothing like having an Apex kit in it.

Welcome to the forum.