HubScore fluctuates, so this is normal. Creating high quality Hubs and participating on HubPages will improve it and keep it high. It also takes a while to get great HubScores from the the time you start on HubPages. If you are doing good things on HubPages, your score will go up over time.
Here is another resource on HubScore.
https://hubpages.com/community/Hubscore put together by darkside and relache's https://hubpages.com/community/Boosting-HubScores