There are so many cases when I find students getting lesser marks while taking classes from well known teachers. Why these things happen? When a student is taking class from an expert but getting problem in understanding the topic. Is it true or something else is behind it?

I have seen so many students complaining about their teachers that they (teachers) don't teach them properly. Sometimes students are right but most of the time students are wrong. No teacher is there who don't want to teach his/her students. I believe all the teachers in teaching profession are willing to teach except few who don't want to do any work.
I have noticed in my teaching profession that there are most students who just do whatever they are asked to do; most of them don't even ask questions related to the problem. I just want to tell students teachers are no God who can find out that you haven't understand the problem solved by the teacher or the topic teacher taught you. There are some points I have noticed and willing to share with you.

 Don't shy in group to ask questions.

Why feeling shy when you paid for it?

I have noticed that while taking classes in groups students feel shy and don't ask questions to teacher if they don't understand; thinking that other students of the same group will make fun of them and will think that he/she is weak.

This is not good practice. If you don't understand the problem and when your teacher asks "Got it" and you say yes (while you know that you don't understand) then what teacher will think? Surely he will think that you are good and have got the problem right. But you know the problem. After this teacher will go to the next level and hence you did not get the 1st level right, you will also not able to get the 2nd level. And problem continues.
What you get after this: A poor result. If you have asked the question to the teacher, you would have got a good result in your hand.

Don't feel shy as in your same class/grade all students are like you, they all think like you and have problem like you. If you ask question, you just not help yourself but also help other classmates as well because there are chances that other students also have same problem like you.

So start asking question and clearing doubts.