I just discovered one cause of liquid shapes not working that matches the symptoms mentioned here. (Well, better late than never!) If you accidentally apply one of the Boolean options (add, subtract, intersect, etc.) to one of the shapes below the top shape in the stack, applying one of these option...

This problem still exists in Moho Pro v13.5. I have been unable to add any characters to the library because of it. I am thinking that there may be some limit to the nesting depth of the layers in the character that prevents this function from working. Is this correct, or is there some other issue?