I had that problem as well, tried to fit a new gearbox in my old M4 as stock one was crap. Snapped the stock selector plate on the new gearbox, then bought a new plate, was just as hard to get in, didn't snap the plate, but was really stiff to change fire mode, so sold the M4 and still got the gearbox sitting here .
Gonna build an M4 from scratch though (apart from the internals as I have all those..).