A Hubber score i have found fluctuates rapidly. Like Powerful said staying active is the key. It also has to do with other things as well such as quality of work,visitors to your work,comments on your work and I believe probably it is even swayed some by what your work is producing $ wise for Hub Pages.
I try to at the very least stick to what I call my 555 plan. Read 5 new hubs and comment (not just good job!,but a real comment with a touch to add to the convo), 5 posts a day on the forum and 5 answers to questions.
Do I accomplish this everyday,NO,but most days I do. Another factor is followers. have you gained any new ones lately? This could be an indicator to Hub pages of the effort you are putting into the site. All of these things put together paint the picture to them of whether you are active enough to warrant a higher hubber score.
That's not to say you are not active,you may just have to adjust your approach to portray that activeness in a better light. Keep plugging away and you will grow,good luck!