There must be some loophole in your communication or skills. I am sorry to say this but it might be true.
I am saying this because you have mentioned that you are always clearing the system test. So skill wise you should be good but way of presentation is also very important.
I have taken more than 400 interviews in my 7 years of career and I have seen many candidates with the same situation.
I would personally suggest you to that you must be having good colleagues and friends in your current organization. Make sure whatever questions you prepare for interview. In free time, ask and answer yourself. Sometimes when you get time, do some interview sessions with colleagues/ friends.
If you do this, you will definitely find your weak points in communication. Then you can changes your sentences and thought process accordingly.
And... some times it might be bad luck also dude... :)
So, I wish your All the best..! for all your coming interviews.