In my form, the icon of combobox cannot show, but other component like button is okay.

What is the possible problem about this?

                <ext:Button ID="button1" runat="server" Text="Login" Icon="Accept" />
                <ext:ComboBox runat="server" Icon="Accept" />
I just directly insert these in my form with same icon, but the one in combobox cannot show.
The starting space of the combobox seems already occupied by the icon, but cursor did not start at the left most .
However the image cannot show.

Thank you!