My DPMS 16" had a FTE problem during a class this weekend. Rounds stayed in the chamber, and had to be driven out with a rod. Gun went down after about 3 mags. Swapped the upper with a known good complete upper and still had the same problem. Ammo was FED. 193, mags were USGI in good shape. Ammo and mags ran fine in 3 other  other AR's. New trigger installed about 2 weeks ago, but prior testing of 500-600 rounds went fine. The only new item installed, but not tested was a GG&G single point mount installed.
I was thinking it was an extracter problem until I swapped the complete upper. Gun was clean and well lubed. I'm stumped.....

Did this "complete upper" include a different bolt from the one in the original malfunctioning gun?

When you had to knock the rounds out of the chamber with a rod was there resistance or did the case start to move as soon as the rod touched it?

The complete upper was just that, complete with bolt and carrier. Just removed my upper and replaced it with the others.

Shell casing wasn't really tight in the chamber, just a little shove. It did show some marks around the rim from the extractor trying to get it out, but after the next round fed from the magazine and shoved it again, the extractor wouldn't get it out.

Still stumped in Texas.