Built my first AR/SBR and took it out to the range this week.  The first two 20 round magazines fired just fine.  After those 40 rounds or so the rifle would not chamber the next round.  It ejects the spent round but does not chamber the next round.  I tried 4 different magazines from friends that were shooting with me and the mags were working fine in their 16" AR's.  The eject rounds did not seem to be ejecting as far as the bushy and dpms rifles that were out with me.  My SBR fires just fine, except it's no fun chambering each round manually!!!!!  

Upper is an 11.5" DPMS knock-off
Lower is a Rocker River
BCG Rock River
Basically everything is Rock River except the upper.  

Is it just not "broke in"?  What can I check on the gas system to determine if it's getting enough to properly cycle?

Thanks for any and all help.  

Most likely the allen head cap screws on your carrier key on top of the bolt carrier came loose.

Possible broken gas ring on your bolt.

Possible sticking cases in the chamber because you are shooting old wolf ammo

Possible gas roll pin never installed in front sight base allowing gas tube to move

One round in magazine fire gun if bolt locks open on empty magazine then you have enough gas.

Thanks for the help.

Quoted: Most likely the allen head cap screws on your carrier key on top of the bolt carrier came loose. Possible broken gas ring on your bolt. Possible sticking cases in the chamber because you are shooting old wolf ammo Possible gas roll pin never installed in front sight base allowing gas tube to move One round in magazine fire gun if bolt locks open on empty magazine then you have enough gas.

I have a new LMT 10.5" upper I put on a RRA lower, and it short strokes with low powered ammo like Wolf or the Monarch green box from Academy.

I checked the things you suggested, but the staking and gas rings were good, the gas roll pin was installed, and the wolf was the new poly coated stuff.

I have an H buffer installed instead of the H2 buffer that many people install. Could that have anything to do with it?