Use
SelectedValueChanged event. it will work.
to idintify the selected value of current combo use
ComboBox1.Text or
ComboBox1.SelectedItem to get value for next combobox.
if you are using ASP.NET then check the
AutoPostBack="TRUE"
The AutoPostBack property is used to set or return whether or not an automatic post back occurs when the user presses "ENTER" or "TAB" in the TextBox control.
If this property is set to TRUE the automatic post back is enabled, otherwise FALSE. Default is FALSE.