Perhaps you need to expand your twitter following/followers more? Are you following several hubbers? Several hubbers also have 'hubber lists' on their Twitter profiles and this helps locate additional hubbers. I see quite a bit of sharing between hubbers on Twitter. Tweets and retweets alike. It is a good idea to include the person's @twitter name when you tweet. This way they will see your efforts and will usually return the favor. Careful with spamming though. You might lose your account and/or followers.
Also, if you simply tweet a link without any kind of description - followers won't see any type of hook or attraction. Again, only tweeting links comes across as spam and not a personal interest.