As we see your tabstrip is not defined inline, so we suppose you are adding the tabs from code-behind. If so, we suppose that you are trying to find a tab too early in the page life cycle.
If your problem persists, I think it will be best if you can open a support ticket and send us a simple running project (incl. your custom skin, CSS, images, DB backup if needed and so on) demonstrating the problem (and step-by-step instructions on doing so). In that way we can reproduce and pinpoint the problems you're facing on our side, understand the logic of your application and provide a solution.
the Telerik team