Teaching Crochet

Just a query - I am both the knitting and crochet instructor at my store. I've noticed recently that trying to teach crochet seems more difficult to pick up then knitting. My last three students did not last the two and a half hour course. Somewhere between one and a half and two hours they are exhausted. I'm wondering if my presentation is wrong and if others have found a way to encourage the students to keep trying a little longer. I don't want to push because this is supposed to be fun, but I feel I'm not giving these students value for their money.

Any ideas?