This description will probably make sense to an expert- I suppose not
to anyone else!

Anybody incounter this before??

Yes, that mechanism is called a Geneva wheel. and it's most likely that
either the first or last "washer", is locking up and not rotating due to
grease congealing and either locking it to the front or back plate or to
another wheel and 2 rotating at a time.

I have a couple of those LMO's if all else fails, but expect that as the
other poster suggests, a drop of WD-40 would cure your problem.

Regards
W4ZCB