ASPNet AJAX MaskedEditExtender Mask not showing on TextBox

I am using the AJAX Toolkit MaskedEditExtender to format some of my web form's textboxes.  When I load the page and start to enter data into the textboxes, the masking doesn't show.  Any ideas on why this is happening?  I've included my page code below:

  <asp:TextBox ID="GrantExpendituresAmount" runat="server" Text='<%# Bind("GrantExpendituresAmount") %>'
                            Width="450px"></asp:TextBox>
                        <ajax:MaskedEditExtender ID="GrantExpendituresAmount_MaskedEditExtender"
                            runat="server" CultureAMPMPlaceholder="" CultureCurrencySymbolPlaceholder=""
                            CultureDateFormat="" CultureDatePlaceholder="" CultureDecimalPlaceholder=""
                            CultureThousandsPlaceholder="" CultureTimePlaceholder="" Enabled="True"
                            Mask="$999,999,999.00" TargetControlID="GrantExpendituresAmount">
                        </ajax:MaskedEditExtender>