The scientific name is your hibiscus is probably Hibiscus rosa-sinensis, since they're the ones that are trained into a tree shape.
Being tied shouldn't inhibit it's growth as long as the string isn't too tight. The clay pot is fine also.
It could be root bound. I know mine don't bloom as much when they're root bound. If that's the case, you can either pot up or do a root trim on it.
As far as making it bushier, I don't know if it will ever revert back to a bushier form, but perhaps pruning back some of the bigger branches may induce growth near the bottom.
The below site is where I get a lot of information on blooming, pruning, etc.
Good luck!
Barb

Here is a link that might be useful: Queen of the Tropics

Barb I want to say thank you for sending the Queen of the Tropics link. Now I can stop my never ending questions to a friend. I wish I had seen this link sooner. I think everyone new should save this link. Cj