This is less likely a result of your build skill or even stacking tolerances than TNW's design. Your experience is common to many, if not a majority, of semi-auto MG34 owners. I attended a re-enactment some years ago, and three-fourths of the TNW semis exhibited this same problem, so it's not just home builds.I usually get about 4-5 rounds off before I get a failure to fire. Charge it, pull the trigger and Ill get 4 or 5 more.
Though innovative, the TNW design isn't robust. I mean, it relies on a little ring to be flipped by an interference plate driven by a tiny spring. It's not particularly positive engagement, as compared to a standard hammer. I'd LOVE to see a design with a hammer; while I've not asked Brian at BRP, I presume he had looked into this prior to to his XM34 (or whatever it is). There simply is insufficient space in the grip for a AR15 hammer, and the bolt's firing location makes such a design, how to say, not aligned.
The befit too of the TNW design is that it's ATF approved; and of course you can order the "rough casting" parts rather than brew your own. So everyone uses what's available, despite the recurring failures, and like Scott are almost satisfied to have an incredible piece of history.