I know this may not be a very 'popular' opinion with you as a parent. However; it may be because your children are 1) Choosing not to listen/learn effectively. (Most) Teacher's aren't just there for the 'pay check', I mean for the amount of work in preparing lesson plans, admin etc you could be paid a substantial amount more. With the new syllabuses; some mathematics teachers alone are responsible for having to teach four syllabuses which will obviously require a lot of lesson planning etc.
Teacher's do attempt to teach understanding in mathematics, but they face a wall of cognitive challenges, as student's are 'forced' into doing a subject which many 'hate'.