Individualism...is it bad?

As I get further along in my practicum, an issue in teaching reading comprehenstion to adolescents presents itself to me over and over in the classroom. In previous education classes we're stressed how all students are different: all students learn differently, study differently, understand things differently. We've talked about how we can try to cater to different learners - visual, audio, kinisthetic. But all of this still seems to barely have an effect on the fact that, especially in the ELL classroom I'm helping out in, every student is at a much different knowledge level then the other.

How do you cater to every student's needs in the classroom?

Even yesterday at practicum, we were doing a root word activity: the one where you write the root at the bottom of a picture of a tree and then use different suffixes and prefixes to create words for the branches. The student I was supervising, although understanding the concept of what a root word was, was unable to figure out how to construct new word, even with both the teacher and I trying to explain it to him. Even when using the dictionary for help, he still failed to spell things correctly, or know the definitions.

Scenes like this are discouraging, especially when you look around and realize that each student is having a different kind of problem with the assignment, and some have already completed it no problem. So, once again, my question for you is this: how can you help students individually in the classroom?