I've had built for awhile now a 9mm AR SBR with a Hahn dedicated block, CMMG ramped BCG, and a CMC 3.5lb FCG installed.  I've been having issues when shooting every 3 or 4 rounds of light primer strikes on the cartridge resulting in a click and no boom.  Then I eject non-fired round next one chambers and 3 or 4 go off then click again.  Is the CMC trigger not compatible with my setup?  I have changed the firing pin and spring with no success.  Bolt face rest's even on end of barrel with a fresh 9mm round in the chamber.  Should I try and put a mil spec FCG in and see what happens?  Any help would be appreciated.  I've read a review on Brownell's that said CMC isn't compatible with a 9mm setup is this true?

Softer primers (federal) or a heavier hammer spring
Will fix it most likley.

Forgot to mention this occured when using S&B 115gr and Magtech 9mm 115gr ammo all brass cased.  Since this is a one unit aftermarket 3.5lb CMC trigger I don't think I can change the hammer spring.  I'm thinking I should put a mil spec ar15 FCG in and see what happens to rule out the FCG issue I think it is.  When the rounds don't go off, after I cycle the bolt to extract the un-fired round there is a dent in the primer but a faint one at that. Maybe the hammer isn't being locked back due to it's different size in comparison to a mil spec hammer?

I would suggest neither of those.
Try a mil spec trigger pack,
Quick question though
What buffer are you running? I ask as I might have a different remedy

I'm using a standard Spike's buffer kit with a Spike's 9mm buffer.

Okay. I'm sure that buffer has extra length built in, that saves our bolt catches right. It looks a lot like a slash heavy buffer.
Take a standard carbine buffer, use one as heavy as you have. grab 8 quarters and drop them down your buffer tube. Install spring and carbine buffer.
Go give that a shot with your cmc trigger.
I have a feeling your not getting enough travel with the extra long buffer.

If that ends up working you could try shaving down the bumper on your spikes 9mm buffer.

Never thought of that (will try on my next range trip), but how would you explain it happening after 3 or 4 rounds fire fine then the next round gets a light strike.  I used the quarter trick on my first 9mm build about 4 years ago with a T2 Spikes, then I got a Spike's 9mm buffer and it was solid all the time with a standard FCG after taking out the quarters.  I'll try the T2 with the quarters next range trip with the CMC.  If that's a no go then I'll pull the CMC out and swap for a standard AR FCG and see what happens.  Thanks.

I'll bet it's the lighter hammer spring in the CMC trigger pack that's giving you problems.  The hammer spring contributes to trigger pull weight, so really light triggers like the CMC will have lighter than standard hammer springs.  A stock FCG will almost certainly fix this for you.

Thanks I was thinking it was the CMC's hammer shape or the drop in itself.  When I hit up the range I'm gonna try the quarter trick for the hell of it, then replace the FCG if it fails to work.  Thanks gents.

I know it's been awhile since this topic was active, but I replaced the CMC with a mil-spec FCG and all is well no malfunctions or light strikes.

If you still would like to have a lighter trigger pull you could try a set of JP reduced power springs with a standard mil spec trigger. That's what I use in mine and it works well with factory rounds and reloads.

I just might try that I have some spare springs from a Geissele SD3G trigger might try those out see how it runs.