If you’ve been an active and helpful member of the community then there isn’t really much you’re doing wrong. You should try to build your reputation and make yourself stand out from the other active and helpful people on the server if you’d like an admin to consider you for the helper role. Another key thing is patience, as this can definitely take some time. You don’t need the helper rank in order to be helpful, and if you are doing it to get the rank then you are doing it for the wrong reasons and your goal might prove even more difficult to achieve.
Have a look at this topic for any other information you might need, best of luck. 