I hope you don't this is too non-partisan but let me try and help. The parts that fell off were they just stuck on or did you score and smooth to make "one" piece? WetSet requires scoring and molding to make it work the way you want. It does not work like Model Magic. Also, leave it in cool water for at least 4-8 hours depending upon the thickness.
I have heard many sucesses from teachers using it. We will be putting a new "trouble shooting" guide soon into the five pound containers of WetSet which should help. You can also call 1-800-CRAYOLA. I hope this helps.