1) Study material provided. It simply doesn't meet the learning criteria. Students are feed with so much of abstract stuff as a result only those students excel who have a better memory retention capacity of abstract information and those who work too hard for it. But such a hard work is meaningless, as you will forget such information after exams are over.
2) Not teaching kids from where they are.
I will explain it with a concrete example.
While teaching chemistry, one of my students was so fast in holding information and doing the problems, while other was slow. The reason for her/his slowness is not that he/she is less intelligent, it's because (s)he is taking more time for processing due to lack of previous foundational information.
When (s)he does the processing, (s)he has to take the account of many kinds of stuff that (s)he has not learned before. For example, while teaching chemical kinetics I found out that (s)he does know the even the BDMAS operation of maths. Chemical kinetics chapter is math heavy, but I tried to explain everything with concrete representation, but still students were unable to do the problems or reach the solution accurately because it requires so much practice of math fundamentals before doing those problems. But the good thing is that they have understood everything that I taught.
From this, I learned that you can explain stuff catching anyone from the street if you are explaining it with the simplest way possible, so that (s)he can understand it.
Similary, while teaching redox reactions, one of my students was too slow, because (s)he doesn't know about periodic table much. What are the electronegative elements? Also (s)he was making the error in calculation while solving for x. Changing values from LHS to RHS or vice versa problem that most students face while solving an equation.
So the fast student needn't have to spend so much time in doing because her/his brain is already trained for the fundamental stuff that is required. So (s)he requires less processing to reach the solution.
So the message is "New information is built over reusing of previous information" For example, one has to know the alphabets to write words, words and grammar rules to write sentences.
Update soon with the complete Idea.....