Why do some students struggle with math?

Daniel Willingham:

Illiteracy and its costs to individuals and to society has long been a focus of concern in public policy. A corresponding lack of ability in mathematics–innumeracy–has received increasing attention in the last few decades. The ability to use basic math is more and more important as modern day society grows more complex.
Some children have a problem in learning to read that is disproportionate to any other academic challenge they face. Some children have a corresponding problem with math. For some reason, the ideas just don’t come together for these students.
In a recent article, David Geary (2013) reviews evidence that one cause of the problem may be a fundamental deficit in the representation of numerosity.
Geary describes three possible sources of a problem in children’s appreciation of number.