For the most part, where I see problems with B/C's, is the gas key leaks to the top of the carrier.
In off cases, it may be the carrier itself that the is slightly larger that will not allow the feed lips of a mag to lock correctly in to the mag well with the carrier forward, as well as the taper in the bolt channel not stepped the correct distance back, and tears the hell out of gas rings.
But mostly, the problem I see the most, and which is an builder errors, which is not checking the carrier with key alone in the upper, to make sure that the gas tube is correctly tweaked to enter and exit the carrier key from the start.
So regarding the carrier key is leaking, simple test of holding the bolt inward in the carrier, CLP around the base of the key to top of carrier on all side, then about 100lbs of compressed air into the front of the key to make sure that the key is not leaking to the top of carrier.
IF the key is leaking, then either send the B/C back for replacement, or if you want to fix the problem yourself, will need to get at least new carrier key bolts from the supply, so you can lap in the key to the top of carrier and reattach to solve the leak problem.
As for carriers that are too large, maybe, but most of the time its a problem with either the lower or upper out of spec, and what is causing the problems of the mag not able to lock home on a closed carrier instead.