Hi there @Minimal-Studio and welcome!
The way I see it, it’s kind of hard to tell you with words what is the secret of approving. There is one simple thing I would suggest you - listen.
Really, just pick up a genre you plan to write in and listen thoroughly to the tracks which were accepted. You might pay attention to certain things: arrangement of the track, length, maybe instruments used and, of course, the quality of the track, as the most common cause of a rejected track is based on quality issues. When mixing and mastering, you could even take a preview of an item from AJ and use it as a refference track (make sure you are critical enough about yourself when working on quality, but not too critical ).
Also, keep in mind that, generally, you write background music. So your music is supposed to be used in video presentations, slide shows, youtube, commercials and so on. Thus, one thing that I sometimes do is I open videohive, load for example a stock footage video about a happy family and just imagine music that could play in the background.
That’s what I could come up with, @Minimal-Studio. Wish you patience and good luck!