A mark can only be lost for four reasons: the topic was not known (knowledge), the topic was known but the wrong approach was chosen (method), the approach was right and something slipped on the way through (arithmetic), or the question was never reached (time).

Only the first of those is repaired by teaching the topic again. That is the reason a capable child can revise for months, be taught conscientiously, and watch the grade sit still — because revision, and a great deal of tutoring, treats every lost mark as a knowledge gap.

Everything in this section follows from that. It is also the reason a tutor should establish what is actually going wrong before quoting you for lessons, and why you are entitled to ask them to.