when not to cancel a lesson

I encounter this one often. Someone decides that s/he hasn't practiced enough over the week (or whatever span) and consequently decides that canceling the upcoming lesson is the best move. It's anti-intuitive but this is exactly the time to take the lesson. Why? Because only about 0.001% of people (and that's a generous estimate) magically start practicing the next week all on their own. The lesson can inspire, hopefully: if not a change is in order.