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.