We'll need a bit more information, as it could be a number of things.
Two that jump out as the usual suspects are waiting too long to cut and using something that is too thick, which then snaps the soap toward the bottom rather than cutting cleanly.
What is your recipe, how are you doing it (slow cooker/crock pot, leaving it covered, how long is the cook, how hot, do you stir)? What do you mean by "falls apart"? Do the bars cut well but then crumble when used?