This is normal; the caps expand as they get wet and when they heat up in the oven. You can use a dab of hot glue on the underside of an acorn cap to reattach it to the top of the acorn body. Just make sure you match acorn caps and bodies that are roughly the same size and seem like a good fit. You don't want to glue a huge acorn cap onto a tiny little acorn body; that would look silly.