It seems a lot of the after market triggers are slightly too wide for the trigger box and when there is spring tension on them they twist and jam in it, leading to not resetting.I had the same problem with a UAC gold flat face trigger as well.
I fitted it and found that on full auto the gun worked fine and t he BCG always returned to battery correctly. But lets be honest, we don't use full auto (practicality) on a GBBR.
In semi i found that after every shot i had to push the UAC trigger fowards by about a millimetre for the gun to shoot again after each shot.
I need to strip the box again and measure up for some thin shims to keep everything as aligned and free moving as possible. Will be trial and error but I am pretty well practiced at rebuilding the trigger box by now!