Few factors that may affect your HubScore:
- Low traffic to your hubs (start promoting yourself via digg/ stumbleupon/ facebook/ twitter/ delicious/ rss feeds etc)
- Less or no backlinks ('edit' hubpage and click on 'suggest backlinks' - backlinks affects viewership and HubKarma - and HubKarma affects your HubScore!)
- Little or no activity on your hubs (start posting new hubs and work/ edit your old hubs regularly to show continuity)
- and of course what ssaul mentioned =)
Hope this helps!