which car? drum brakes are quite similar though.
when you put the new shoes I assume you must have pushed pistons in, but just try to push them in further with shoes on... they might just have not gotten pushed back in far enough for drum to go back on, but enough to get the pins through that hold the shoes.
also: did you slacken the hand brake cable at the lever at the front? if there's a bit of tension on it - in particular if old shoes were very worn then it's just out too much for drum to go over.
does the wheel bearing sit properly? you need to push the drum on pretty square otherwise might struggle as well.
that's obviously assuming you put everything back together correctly....
