Why Contents of combobox in the listview header disappers in a page,when back from another page,ie,the rebinded value is not visible

Am having a listview with a combobox in its header with header named city. Contents in that combonox are binded in according to the selected State.My Problem is that when its loaded the values in the combobox works properly but, if i move to some other pages and when am back it to the same page values in the conbobox are not visible,ie,the rebinded value is not visible.why this happens?