Combox SelectedIndex - Do not change

I am using Combox for the months of the year.  The SelectedIndex is working fine if I do not set a default when I display the form.  But if I set the selectedIndex before I display the form the default value do not change when the user select OK.  Example When I set the month to the current month December = selectedindex 11 and the user select August the SelectedIndex is still 11.  This code is working fine in the Windows application but in the Web application it is not changing.  What must I do to get the SelectedIndex to change when the user press the OK button?