If the lock up is intermittant between cylinders but consistant on any individual cylinder, the ratchet on the ejector star could be the problem. In rare cases it could be the cylinder notches themselves.
Clean it well, including the notches, look for burrs and try it again.
I'm thinking cylinder stop if it's not locking up when it supposed too, maybe the hand isn't turning the cylinder, so that will be the little spring that holds it in place.
It's hard to guess without looking at it, but I'd think something in the lock work. I'd take the side plate off and give it a good cleaning then reassemble it and cock it slow DA and SA. Let us know how it acts afterwards.