Duir,
It seems like the problem is with the flare-point recognition. One method i have used to get round this is to:
a. Demonstrated a good flare &landing.
b. Let the student fly most of the approach (for his/her practice) then take control and get the stude to follow-through and say 'now' when they think you should flare. (obviously, jump in if they say nothing!)
This separates the recognition from the handling and can work in some instances. When you think that they've got it hacked, emphasise the
smooth but swift transition to the flare attitude with simultaneous idling of power.
Keep practicing but if it isn't working that day, don't let the stude get to down-hearted or hung-up on it - it will 'click' at some point.
GSS
