ASP.NET Validators should be visible to do validation

Validators are disabled, if they are not visible. They can be not visible, if any of their containers is not visible.
In general, they should be always visible, but the message will be shown only if they will be NOT valid.
Also consider to set BaseValidator.SetFocusOnError=true.
In big pages use ValidationGroup combined with CausesValidation property set to true.