OK, so no inertia lock.

Progress anyway!

I actually owned one of those for a while, but I accepted an 'offer' an sold it. Mine worked with anything. 70's gun.

What do the empties look like? Can you see any bulges in the rims like from excessive headspace or expansion into the extractor groove?

It's very weird that the problem would just 'go away', which is why I suspected the bolt wasn't unlocking.

"The price of good shotgunnery is constant practice" - Fred Kimble