Number Troubles

This morning, my second son was struggling again with his maths Kumon. He is working on subtraction. He will seem to 'get it', but then the next day revert to some strange method of doing his sums, which shows no understanding of the numbers on the page or the process of subtraction. It is frustrating - for him and for us. I am sure that in school he would be moved on because there is pressure to get through the curriculum that needs to be covered - whether the learners are ready or not. My husband, in his work both in schools as a secondary maths teacher, and now as a Kumon instructor, knows this to be true and sees the result. Children are grappling with higher level concepts, without having a good grasp of the foundations. These sure foundations can only be built by practice and repetition, which Kumon does provide.
Rather than moving our son on, we decided he needs to go back and repeat subtraction problems he is confident with, building up again to the harder sums until he 'gets it'. And I am confident that, with practice, he will and then he will feel chuffed with himself for having persevered and cracked it. Our eldest son is facing a similar problem with his long division. He can do this now, but he needs to be able to do it more quickly, to be 'more fluent' at it before he moves on again.
People might look at our second son, and wonder why he doesn't know his times tables yet. Well, he will get there, but I can see how fluent his addition and number bonds are having followed this approach, and we will continue to move at his pace - not at the pace dictated by others.