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.
Thank you David. I think I get what you meant. So, can I reword it this way? "Despite my efforts, sometimes, I feel that my English knowledge has not been improving significantly lately". – jeysmith – 2013-08-05T19:22:20.917
It's not clear what the following note is about, since neither the rephrased sentences uses have not: "Here have not has a sense of a process or of work." – kiamlaluno – 2013-08-05T20:36:48.580
@kiamlaluno thanks - I'll reword to make that clearer. – David Hall – 2013-08-05T20:45:21.260
@jeysmith - yes, your new version is clearer. – David Hall – 2013-08-05T20:53:42.817