Maybe the recipe needs to be adjusted a little. If the melt were slightly denser or harder it might not crack when you remove it from the mold. I would suggest adding more Shea Butter (If your Shea is very hard) or maybe adding a small amount of beeswax to the mix. Just be careful not to add too much wax or it won't melt in the bath water.
Also, depending on your recipe, they may harden up if they air-dry overnight. If you have a lot of powder in the recipe, the freezer's humidity may be causing them to crumble. I made some melts once from salt, starch and oil that hardened after being exposed to air for about 24 hours.
Good luck! I hope you can find a way to firm them up.