I knew they were capable of understanding what I was teaching, and I even made it very clear to students that if they are having trouble with the homework they can bother me and I will help them through it, and I will spend whatever amount of time it takes. A few students actually took me up on that, and they really did improve as a result, but some of the students simply seemed content on failing.
I take it as a personal failing; if a person is smart enough to pass my class and didn't, then I didn't do a good enough job making it interesting.