This happened to me recently. Try putting in the expander ring FIRST, then the indexed top rail, then the bottom rail.
And you mentioned that the pistons are spotless. But are the grooves also spotless? You probably know this already, but you can break an old ring, and use it to scrape out the grooves (using the "unbroken" end of the broken ring). Also it's a good time to poke out any gunk in the relief holes that are in the oil control groove, if necessary.