It is hard to discover when a protected sense is draggable and, if it is, what is allowed. A couple ideas that might help:
Add a locked icon or different color when a sense is the final sense in a protected word and thus cannot be moved.
If a protected sense is movable (not the final sense in a protected word) and something illegal is attempted (dropping in another sense or the trash), pop up toast to explain to the user why nothing happened.
It is hard to discover when a protected sense is draggable and, if it is, what is allowed. A couple ideas that might help: