I'd suggest making sure nothing stupid is happening like something being stuck down the muzzle/run through the bore such that occasionally a bit is left to act as an obstruction.

Second,I'd make sure thate is no wad issue that could contribute to shot balling.

Third, I'd look for any coating on the shot that could contribute to said shot not flowing and/or having an unusually high friction coefficient for surface to surface contact.

Forth, I'd check shot size inuse and make sure it is #6 or smaller as larger shot is more prone to bridging.

Last, if two, or more, proof loads are being fired, I'd check for shell bases coming loose and lodging in the choke section.

If one of those doesn't get it, I'll have to think some more.