Did you replace the shoes as well? From the way I read your post, it sounded as though you just replaced the WC.
So, I am guessing that one or both of the shoes has slipped into the backing plate indentation or groove. If they have, then the shoes are spread apart to far for the drum to go back on. Or, the backing plate has a worn spot that has developed a large burr. If you are pushing on the shoe to compress the WC, then all your effort to do so would explain that they slipped into the outer groove.
Stupid question, did you open the bleeder to allow the WC to compress?
This photo gallery that someone did on the assembly of drum brakes is nice to look at for a stare and compare.
http://www.pbase.com/nufsed/drumbrake