I am trying to add a **FilteredTextBoxExtender** in a `textbox` but there's no smart tags from which to choose `Add Extender`. I already removed the ajax toolkit in toolbox and added it again. I tried also to drop the extender into the source view and manually add it to the textbox but still no effect.

Before, there is a smart tag of adding extender but now I can't find it anymore.

What must be the problem on this?

Thanks in advance.