You deduced the issue correctly. It’s the TTI hammer spring causing you grief. Go back to oem and your issues should go away. I’ve never known anyone who’s M4 would work with those pieces of crap in the pack.
Was there small dents in the primers of the shells that didn’t go off? If so, probably light strikes from the light hammer spring. If reliability is a concern, it’s best to not mess with the trigger on most weapon systems.
So for anyone else wondering this in the future...the Comfortech stock (61176) doesn't have the nubs but it came with a "B" shim, "SX/DX" cast change shim and an "A-B" 50/55mm drop lock plate in the box. Once I figured out how this goes together, it mated right up. Definitely like the feel of the Comfortech stock over the pistol grip.