This confuses and frustrates they hell out of me too. Here is my theory. It is a combination of things: Is your hub 600 words or more? Do you have original content, sometimes a picture download from the web you pasted can raise a flag. How many keywords do you have that are relevant? This is a biggy. Don't throw any word as a tag, phrases are better. Think of it like this, how would look up your topic? How involved are you in the hub community? Again this will affect it.
And it is good to let the hub marinate for awhile let it get to a 60 or so, and if it doesn't move at all, then go back and fiddle with it.
Google, Yahoo, Bing etc. can be testy and hormonal what they like so one day it's good, the other day your are the anti christ.
Buy a magic eight ball, when in the ultimate doubt use that.....