AJAX :: Can't Show ValidatorCallout?

I'm using Update Panel.It contains a modalPopupExtender that use to insert new username.When i click on button, it will check if username exist .If username exist ,I want to ValidatorCallout show.But it doesn't show.here's my code

<asp:CompareValidator ID="CompareValidator_UserName"
ControlToValidate="txtPopSkillName" ValidationGroup="SkillAdd"
Operator="NotEqual" runat="server" Display="None"
ErrorMessage="UserName existed" SetFocusOnError="true"
></asp:CompareValidator>
<cc2:ValidatorCalloutExtender ID="ValidatorCalloutExtender_UserNameNotRepeat"
TargetControlID="CompareValidator_UserName" runat="server"
Enabled="True" >
</cc2:ValidatorCalloutExtender>
buttonClick_event
bool IsExisted =CheckUserName(txtUserName.Text);
if(IsExisted)
{
CompareValidator_PopSkillName.ValueToCompare = txtUserName.Text ;
ShowPopUp();
}

Ps:When I click one more ,Validator will show Have a solution to solve?