There are several hubs about how to conquer writer's block. Maybe reading them will help.
If I were you, I would take a look at the 28 I have, and see what kinds of unanswered questions I can fill. Check the statistics to see what keywords people are using to find your hubs. I had written a hub about genealogy, and someone had used "genealogy questions" to find my hub. I knew that hub didn't get them what they wanted, so I wrote one that would!
Look at your hubs that are doing the best, traffic wise, and write more about that topic.
Otherwise, just think about what you do day to day, and things you want to learn about. There is always something going on in my life that is hub worthy!