I am typically a "good test-taker", but for the sake of argument, I do believe there are "bad test-takers".
1). There are people as you pointed out who have poor reading or comprehension skills, but it can go beyond that...
2). There are people who have difficulty with "critical thinking", that is to say, they have difficulty with their reasoning skills. (This is the one that is hardest to fix, especially as an adult)
3). There are people who experience severe anxiety from tests in general. I guess if you had a time machine and could go back and fix the right trigger that began it all, or could delicately re-wire someone's neural pathways, then this could be a simple fix. However, there are some medical solutions that can help, certain medications, certain dispensation for lighting in the room, or other factors to affect testing anxiety.
4). There are people who have difficulty focusing. The exam starts and all they are concerned with is the sweating of their palms, the temperature in the room, the barely audible ticking of the clock, how their dry erase marker isn't working very well, what the person seated near them is wearing...
Regardless, there are some awesome EMTs that I work with who know the book information for medic school down cold, are awesome in the field under pressure, can reason through long case examples, but can't pass the test, and have difficulty passing written tests in general. It's not just as simple as saying, "Work harder to get smarter." There can certainly be other factors involved.