CheckBox click event?

I have some custom filtering on a grid I'm doingwith comboboxes and AJAX. Each combobox is linked to the grid the the rad ajaxmanager like so:

                    <telerik:AjaxSetting AjaxControlID="ddlXXX">
                    <UpdatedControls>
                        <telerik:AjaxUpdatedControl ControlID="grid1" LoadingPanelID="RadAjaxLoadingPanel1"/>
                    </UpdatedControls>
                </telerik:AjaxSetting>

I'm calling a GetDate() method that returns a data table to bind the grid to the data source in the page_prerender event.  All works well, except for when I uncheck a checkbox in the combobox. If I check a checkbox it causes the post back and the GetData() method is called, but if I uncheck a checkbox the GetData() method is never called and the filtering isnt refreshed. Am I missing something in the Page LifeCycle? Why wouldn't the Page_PreRender event be called when I uncheck a checkbox inside a combobox? Should the DataBind be called at another point in the page's lifecycle?