OK, new scenario:
Take an average ability student who tries their hardest to do well in a class but cannot break a "D". I believe that the problem lies not in their ability to do the new material, but their mastery of simple concepts associated in the course. My theory is that if that person were to go far back as possible to the basics, and work their way up applying their maximum effort, they would not have a problem in the class they are struggling in.
For example: Someone mentioned their sister not understanding calculus for the life of her. I personally had a hard time in calculus, earning a B, for lack of effort as well as not really mastering basics, such as trig, etc (bad trig teacher in high school).
Hypothetically, I would bet money that if she went down say to Algebra, and mastered that, followed by geometry, trigonometry, precalc, etc. and finally worked back up to calculus, she would understand it and do well. Because it seems quite evident that the inability to understand a calculus concept lies within the context of not understanding a more simple concept. (Like the idea of asymptotes in precalc and limits in calculus.) I think it would be hard to understand a limit without understanding asymptotes! (No offense to the kids sister, just using her as a hypothetical example).
Anyways, this is why I think it depends upon the input of time a student dedicates, however, this is under the assumption of mastery of basic concepts.
What do you guys think of this?