9mm AR-15 problem.

I have a 9mm AR. When I was shopping for one I considered building one and did some research. They need a different hammer and I think springs in the trigger group also.

Maybe short stroking and not bringing the hammer far enough back to catch?

Shooting full power loads?

I don't know if the buffer and spring is supposed to be the same as the ones used for 556, but I would check into that.

Did the upper come with a bolt?

Are you running the 9mm buffer?
If not that would cause the problems you have mentioned.

I would also say the Buffer might be the problem. What weight buffer are you running?

They make a 9 mm weight buffer.

I had the exact same problem with a CMMG upper. However I had a RRA NM trigger in it and it was simply not tall enough for the bolt to re-set it.

I swapped out NM for an ALG Defense ACT trigger and all the problems went away. Don't discount the trigger as your problem yet. See if you can find comparative measurements for your trigger vs. full size mil-spec triggers. The 9mm runs a very close tolerance and that's definitely the first place to look.

EDIT: sorry, I said trigger, but what I'm talking about is actually is the height of the hammer itself. I replaced the entire RRA trigger/hammer assembly with an ALG assembly.