The number is an average of your hub scores. If you are writing hubs quickly and posting, it could be that your hub score has not had enough time to mature - they go up as readership increases, etc.
The other reason might be that you are not getting the traffic you need, or the quality of the hub is not as high as you would want.
The two ways to raise traffic are through backlinking and keywords. If you are not familiar with these terms, put either one in the HubPages search box at the top of the page; there are many good Hubs on the subject.
Good luck, and hope this helps.