Maybe they're just not that smart....?
Think about music. Learning the different scales by memory is important, but if you don't think about them in terms of intervals, and about those intervals in terms of whole-number ratios of frequencies, then you'll (probably) never become a great musician. Multiplication is interesting here, because there are lots of different ways of thinking about it, and when you come to do things like multiplying matrices, you see that it isn't always just repeated addition.
Perhaps they are behind where you were in terms of rote numeracy, but perhaps they have a deeper understanding of numerical objects than you did at that age?