New build with WOA Varmint upper:
New Bushmaster 10rd Mag.
I am getting a FTF every other round and sometimes a FTE.
Especially if I load more than 3 rounds in the Mag.
It is locking back every time on last round.
It starts to load the next round but after it starts up, the bolt slides over the top of the round
and jams in the middle of it. Sometimes it does not feed one at all but the bolt closes and locks, dry firing. then sometimes it Fails to Eject.
But mostly the bolt jams a cartridge in the middle

I had a problem similar to that, which started with new mags. I found the mouth of the case was hanging up on the front lip of the new mags. disassembled, filed sharp lip off, and cleaned with good old clp. 0 problems since then.
maybe this could help you.

New Bushmaster 10rd Mag.

 That's the first thing that I would look at.