I’m not improving my English knowledge significantly.
I have not been improving my English knowledge significantly.
I would like to know which is the best choice between the two sentences above.
I mean that I have been studying English for a while but, sometimes, I feel that my English comprehension is quite low till now.
Neither of your phrases captures the meaning that you want.
The reason is that in both you make yourself (that is I) the subject; this conveys a sense of you actively doing (or not doing) something.
A better phrase would have your English knowledge as the subject:
My English knowledge has not been improving significantly.
The two phrases you gave have a different meaning. They could be reworded to something like the following.
With I’m not improving my English… the phrase I am not has a sense of personal desire. It could be reworded to:
I do not want to improve my English knowledge significantly.
I have not been working has a sense of a process or of work, and could be reworded to:
I am not working at improving my English knowledge significantly.
Source : Link , Question Author : jeysmith , Answer Author : David Hall