I think the most important thing is not to ever turn it into a contest. (No prizes for guessing who's reading Aikido in Everyday Life by Terry Dobson and Victor Miller at the moment!).
The main thing is that you're not failing when they don't do something - it's just that the path is a little uneven at that point for them. Which means they need you more than ever to help, gently, guide them along.. Getting upset, applying Yonkyo, shouting - these things are not the Way...