I had a very similar experience with my first AR9 build. I purchased a FM9 lower from Brownells and the FM 5" tri-lug upper that Primary Arms was selling at the time. I could not put the two together - and I had backed the tensioning screw all the way down. FM was very responsive - they sent me a shipping label for the complete pistol and returned it about two weeks later with a new upper.
The last round hold open worked OK. Most of times it didn't work happened with Glock OEM mags - Pmags seemed to work well. Same thing with the annoyingly frequent FTFs with 147 grain ammo. Alas, those problems were game enders for me - I'm deep in Glock OEM mags and intended to suppress it with subsonic ammo... On the bright side, it was remarkably accurate.
So, I traded up (with full disclosure) for a CMMG Banshee 5". The Banshee has been 100% flawless for over 1000 rounds of all weights, both FMJ and JHP. By flawless, I mean zero failures to feed, fire or extract, and the bolt has locked back on every empty magazine (Pmags and Glock OEM). I guess it better be good, since it was pretty stinking expensive...
Note that I am NOT unhappy with FM - their customer service was solid and I imagine they would have debugged the FTF issue had I given them the chance. But the Banshee opportunity presented itself, and I haven't looked back...
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