Engelbert Schmid Bell conversion problem

I have a Paxman 70L that was converted to detachable bell with an Engelbert Schmid ring about a year ago. Conversion was done at Houghton in Texas by previous owner, so I have no reason to think that there was any problem with the installation.

I have tried several bell flares in the last couple of months and, while a couple screwed on all right, a number of them bound up quickly and would not attach to my horn. As far as I can tell, all the bells I've tried - including the 2 that I use on my horn - screw easily onto my friend's Schmid triple bought new in 2000.

If anyone has any experience with this type of difficulty, I would be interested in your thoughts on the matter.