In other words, they never memorized their multiplication tables! Many times I have seen a student do poorly due to a weakness in basic math facts they should have learned in third grade.
For many middle-school children, math is a real challenge. It is at this age that the concepts begin to get more abstract, and work goes beyond "concrete" ideas like adding and subtracting. It's no surprise that at this age, math can become something the student avoids, or begins to dislike.
When a child learns to relate math to everyday questions, he will be great at it from the simplest addition all the way to trigonometry. To convert percentages, decimals and fractions is thus one essential skill.
Homeschool worksheets fortunately will not over-tax your budget. There are many places where you can get them at extremely low costs. In fact, several websites offer printable worksheets for free.