Clean your cylinder really good, and look through the chambers with a light. You may have tool marks in a couple chambers causing the expanded cases to grip the chamber walls tighter than the other chambers.
If that's the problem, a couple quick passes with a Dremel tool, and some polishing compound will likely fix it. You won't need to completely take out the marks. Just polish them to knock off the rough edges, and your empty cases should come out easier.
I had to do a couple chambers in my 357 Blackhawk that way, and now it ejects with ease.