It would be easier to help you with this problem if you posted your project here. Without access to it, we can really only guess what is going on.
That said, the most likely issue is, your image origins are set wrong. Many times, when a new user creates a character from a sprite sheet, they don't realize they have to fix their image origins. If your characters origins cause their collision box to push down into the solid floor, it will cause them to fall through.