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.
Hopefully this has inspired you to develop exciting and engaging review worksheets for your class when needed and your students achieve as much as they can when it comes time to test.
Subtract 1-, 2-, 3-, 4-, or 5-digit number from a 5-digit number
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.