This is exactly what my Zen master used to talk about, "a student will not understand, until he understands it for himself." (Jibun ga ki ga tsuku made, wakatte kurenai.) I used to think, "how can he understand unless I explain it first?" Sometimes, no matter how much you say or explain, they just won't understand at all. . . . . .
Nantembo, a Zen master, used to say, "If you answer me correctly, or get it wrong, you still get whacked with my staff either way!" You don't have to get whacked, but even Zen masters had a hard time teaching and sometimes, it is not just the words - one must ask, "where does true understanding come from anyways?"