C# WPF app combobox pulldown not working correctly

I have a C#, WPF app that is having a problem with the combobox pulldown. It on some occassions is not displaying either the pull down choice chosen or the correct data. I am told that i need Bind ComboBox to an ObservableCollection, so UI will get automatically updated. I do not know how to do this or locate this area. here is some of the code: I think it should not take too long, I also will have other such work as we are promoting this [login to view URL] someone to do this Friday AM USA Eastern time. [Export] class ScheduleViewModel : TabViewModel, ISelectedContentTab { // private readonly String[] _assessmentType = new String[] { "All", "INCOMPLETE", "COMPLETED", "COMPLETED LATE", "INCOMPLETE LATE", "NOT STARTED - LATE", "NOT STARTED", "SUBMITTED", "SUBMITTED LATE" }; //pk display ALL as default in detail tab. // public ScheduleViewModel() : base() { DisplayName = "Schedules"; StartDate = [login to view URL](-6); EndDate = [login to view URL]; GroupDataCollection = new ObservableCollection<GroupData>() { // here is the combo box [login to view URL]() , new GroupData("Criterria" , [login to view URL](StartDate,EndDate,(s, e) => { StartDate = s; EndDate = e; RefreshData(); }) , [login to view URL](UnitChanged) , [login to view URL]("Assessment", "Assessment", "Select Assessment to show.", _assessmentType, (type) => { AssessmentType = type; }) ) };