I have this problem, I've got a navigation-based application, and on the one of the detail view i need to have UITabBar, which will display some UITableViews. Since apple documentation says "you should never add a tab bar controller to a navigation controller" it make quite a problem, i've found this sample: link text, it's working, but after picking one of the table view, the UITabBar disappears.
Now that you are not using a TabBarController for showing the tableviews (as mentioned in the link), have you made sure that the table views or any other views you are adding when a tab is tapped are correct size?
You are adding a subview or bringing it to top so the table view is probably covering your tab bar.