Math is a subject that just doesn't come naturally to them, and it takes extra work and study for them to understand. This is doubly true for more difficult concepts like multiplication and division. Occasionally, a difficulty in understanding math can mean something more such as a learning disorder.
It's by no means the only marker, but being unable to follow class and concentrate on homework can mean a possible learning disorder. People with dyslexia may have problems properly seeing the numbers on the paper or blackboard. When they try to solve the equations, they can't make them out correctly.
Many learn to fake it for writing and other classes, but its difficult to fake math problems. If the teacher isn't observant, then the student can get lost in the shuffle and placed in remedial classes because of an undiagnosed disorder. With proper training and education, people can lead perfectly normal lives. Math may always be difficult for them, but they will be able to understand the concepts.
I know many people who have faced the difficulty of living life with a learning disability. They have worked hard to not let it define who they are. It's simply something they have to deal with and they have learned to deal with the problem.