Could just be that either the entry to the key is worn out, or was over sized to begin with. Hence a quick check with a gas tube will tell you if the entry to the key is too large from the start.
Also, depending on the age of the carrier, a while back there was a bad batch of keys that the bottom passage exit of the keys where not aligning correctly with the top entry port of the carrier. In some cases, you could en-long the bottom of the key exit port to align the two ports, while others where so bad that the only solution to the problem was to get a correctly produced key instead. Hence the trick here, was to catch the problem before the key was installed to begin with.
Lastly, double check the bolt tail passage in carrier as well, hence could be over sized as well. Here with nastly gas leak, if say a rifles gas port is over sized, even with the excess leak out the tail of the bolt, the carrier would work fine in it.