These students have weak immediacy. This means that compared to other students, they are slow to come up with the right answers to basic math problems like 6 x 8, or 35 divided by 7. While other kids have the answer stored in their memory, these kids do not. This causes big problems when they try to work through the more complex problems they encounter in junior high math.
As a middle-school math teacher, I have seen many students who do understand abstract concepts, and even enjoy learning about things like Pi and the Pythagorean Theorem, but who appear to be unable to do so due to a weak grasp of math fundamentals.
Subtract 1- or 2-digit number from a 2-digit number with/without renaming
There are several standard exercises which train students to convert percentages, decimals and fractions. Converting percentage to decimals for example is actually as simple as moving the decimal point two places to the left and losing the percent sign "%." Thus 89% is equal to 0.89. Expressed in fraction, that would be 89/100. When you drill kids to do this often enough, they learn to do conversion almost instinctively.