One of the great mysteries of hublife! I think you probably need to publish more quality hubs and provide thoughtful comments on other hubs (not just answer questions). My hubscore has stayed in the high 90s for weeks, and I definitely notice publishing and commenting have a positive effect (and that's most of what I do!)
I have no idea whether having questions voted down affects your hub score - I hope not, because it would discourage people being open in their answers. Good luck!
You can help the HubPages community highlight top quality content by ranking this answer up or down.