laziness versus stupidity?

so i am grading exams i just gave to my classes on a topic that i am required to teach out of what i would consider to be an appropriate order. now, i gave them what i would consider as much help as possible...let them take the test over two days, gave them formulas that they need to help as they took the test, gave them hints, and i am giving partial credit for showing work on a multiple choice test...and yet somehow there is just a great dispairity in the test scores...it is painfully aparent those students who either got it, or worked to study after seeing the test. in one class my range of scores is 30 out of 100 to 99 out of 100. is it my fault? did i not teach them? or is it a matter of them not trying? do i need to do something more? or are they just totally unintelligent? am i asking too much of "average" teens?