Harry didn't seem to realise he had a bottom part of his hutch for a while. Eventually he was lured down there by his hay - I had put it in a hard plastic cup so he had to work to get it out (mental stimulation and all that!) and because the cup could roll, after some time it fell down the ramp. He wasn't sure at first but his stomach soon won over his uncertainty and he went down the ramp after it.
If I was going to teach a bunny about ramps again, I think I would put a bit of a treat food on each 'rung' and hope they would follow it down.