Tubb CWS will add 0.080" to the length of the BCG. If your buffer face isn't chewed up, that's not the problem, though. Post a pic of the buffer face for confirmation.
Stack a couple quarters in the receiver extension, then insert spring, buffer, etc. See if that solves it. If you can't lock the BCG back by hand, then remove one of them. See if it works.
2nd time I've seen this with the PWS extension, in just a couple days here.