I don't know whether they don't recognize the
sink or just can't translate the recognition into action.

Thoughts:

1) A good demo on your part every 5 landings or so.

2) Power off stalls at least teaches the muscles the right motion.
Point out the similarity.

3) Fly down the runway a foot or so off the ground with power to
teach them where the ground is. OK if the mains touch occasionally.