I was shooting my ar today, which has an adjustable gas block which I already have set to what I thought was a reliable amount of gas. I already shot 150 or so rounds without a hiccup the other day, and today I shot 100 rounds. during the first mag I had one shot where the trigger did not reset although it cycled fine. The trigger i'm using is a Geissele match adjustable two stage. I do not know what the trigger pull is but it's light.

I'm guessing it is either under gassed or the trigger is too light but I really don't know for sure. Anyone have an idea what could have caused that?

If it was once one round out of everything you fired, then it might have just been a bad round, but I admittedly haven't used an adjustable gas block or Geiselle adjustable trigger.

Since it cycled, my guess is that the disco didn't catch the hammer on the way back, and from the momentum built by the hammer coming back forward it may have blown past the trigger sear as well.

Are you sure it fully cycled when this happened?

It definitely picked up a round and completely seated the cartridge. The bcg looked completely closed as well but I didn't check if it was all the way forward, but my guess is it was. There was a indent in the middle of the primer from the firing pin like when the BCG slams home so it must have.

Alright. Since it fully extracted and chambered a new round I think I'm right. I think the disco didn't catch the hammer, and the hammer sear may have clipped the trigger sear, slowing it down enough to not ignite the primer on the second round, as the hammer came back forward resulting in a light primer strike on the newly chambered round. (Round number 2 in this scenario)

It's either that, or the case never extracted, and it sent the spent case back into the chamber without fully bringing the hammer back either, therefore not resetting the trigger. (This would be a result of being undergassed)

You may have a blown primer rolling around in there.

Scenario one sounds correct. It was not a empty case because I dropped the mag and opened the chamber and retrieved the cartridge. Bullet and all was still intact. The primer did looked pierced more than a normal chambered round.

What could I maybe do to fix this? Maybe increasing the poundage of the trigger?

If it's option number one I think you should really get a new FCG to fix the issue. But I'm also not familiar with adjustable triggers. But Geissele is a stand-up company since Bill is a stand-up guy. They will take care of you, id shoot em an email or an I.M. here on the site

Thanks for the help milspec. I will get ahold of him. Love me some Geissele triggers, I have 5 and the only gun that doesn't have one is my truck gun.

No prob bro. And yep, they do make some high quality triggers indeed. Between Geissele and ALG they have my trigger market cornered.