Most likely the ejector rod is unscrewing and binding things up. This is a semi-common happening with Smith & Wesson revolvers.
The easiest way to deal with this is with a business card and a couple of Popsicle sticks.
You use the stick to wedge the ejector rod so that it won't turn and you partially pull the trigger so the cylinder bolt starts to retract. You then slip the business card between the cylinder and frame so that the cylinder bolt can't lock the cylinder.
Then you gently turn the cylinder counter clockwise (I think that's the proper direction) to screw the ejector rod back in. Probably no more than a quarter to half turn.
Once you get the cylinder open, you need three or more empty shell casings.
Put those in the cylinder, then with a PADDED pliers you hold the rod and turn the cylinder to get it good and snug.
Chances are eventually it will do the same thing; that's when you employ a little Loctite to hold everything in place.