how to use updatepanel

Question

i'm using updatepanel on DropDownList and trigger on CheckBox

and another updatepanel on another DropDownList and trigger on the previous DropDownList so when i check the CheckBox the first DropDownList change without refreshing the page good, but when i select new index from the first DropDownList after i checked the CheckBox the second DropDownList must change without refreshing the page but it won't change can anyone help ?