There is a problem with Validator controls not being displayed in (Ajax) UpdatePanel after another updatepanel performs a partial postback.
i.e. Validators work fine when page is first loaded; then another UpdatePanel control performs a partial postback; the original validator is no longer displayed.
This is dues to a problem with release v1 of Asp Ajax. A patch is to be released to fix this, alternatively compatible controls can be downloaded
See this link for more details
