Calender Extender in user control is not rendering sometimes

Question

Here is the my code:UserCVontrol
 <asp:TextBox Width="70" ID="txtStartDate" runat="server" Style="margin-left: 10px;"
  oncopy="return false" oncut="return false" onpaste="return false" onKeyPress="javascript: return false;"
  onKeyDown="javascript: return false;" />
  <input id="imgStartDate" type="image" src="../Images/datePickerPopup.gif" />
  <ajaxToolkit:CalendarExtender ID="ceCheckInDate" runat="server" TargetControlID="txtStartDate"
  PopupButtonID="imgStartDate" PopupPosition="BottomLeft" Format="dd/MM/yyyy" CssClass="calendar">
  </ajaxToolkit:CalendarExtender>
Here is unable to paste the screen shot output:
In output window even coud not see the default text/alttext of the image of the control