If yes, its have number of meanings. It might be that you are a person of introvert nature, you rarely interact with your teachers, first point. Here may be a gap in between the teacher and student or there is a possibility that teacher might be strict, but second point.

Let us consider one more case, the student is not regular in the classes and he doesn’t know about any of the topics, the teacher ask question to him, but he don’t reply the answer and don’t feel nervousness, why? In your case, point is you are working hard, you are regular in the classes,  you know the topics, the idea but you are not able to express when teacher ask, you feel nervousness, why? On the other hand you can say that you loose your confidence when an expert ask the question, you feel nervousness, why it happen so with you? The answer of it you have, if you see carefully all the things associated with you. 

Its answer is very simple, inspite of attending the regular classes, you are missing two points. These points are, number one your interaction with teachers is limited or nill second and important you are not revising the contents category wise means sequentially.  First point will finish the hesitation of interaction that what and how to tell the answer to teacher, second will develop the confidence. YOU ARE A LEARNER, A CREATOR. TALK WITH PEOPLE FULL OF CONFIDENCE. This is wish of the GOD, you are blessed person, so enjoy the life just bring either one of the change in your life or both. Now its in your hand.