We had a chap here a while ago who was very negative about wwoof hosts in general as he said he'd done nothing but chop fire wood and other 'boring' tasks at several farms. When I quizzed him further he revealed that he'd been at each farm for only a day or so. When I tried to explain that most wwoof hosts don't have time to explain complex tasks to someone who will not be there to practice what they've learnt he really couldn't grasp it.
We go out of our way to do extra sessions with volunteers who are interested (it takes me three times as long to make cheese with the 'help' of a volunteer for instance and my partner regularly spends a session explaining the technology behind a RAM pump) but these are things that we can only do once all the essentials are covered. If we don't have time to weed the potatoes, and the crop is poor as a result, then we don't eat potatoes next year. That's how simple it is. The vast majority of volunteers understand and go out of their way to help make this project a success, but the occasional one makes me shake my head in despair.
I've been hosting for long enough now to recognise that not every volunteer will be a good match for every farm, just as every farm won't be ideal for every volunteer. We can only do our best and try to communicate our expectations in advance.