Why does my dropdownlist inside my gridview not populating?

I have a grid view control to which an edittemplate has two dropdownlist control called DropStartTime and DropEndTime.  For both controls I want to fill with time values I use the following code to do so:
Public Sub GenerateTime(RowIndex As Integer) Static Dim ComboStart As DropDownList Static Dim ComboEnd As DropDownList Dim ItemList As ListItem Dim TimeInterval As Integer Dim IntervalleCalcul As Integer Dim NombreIntervalles As Integer Dim HeureCalcule As String Dim Laps As Integer ComboStart = GridDetails.Rows(RowIndex).FindControl("DropStartTime") ComboEnd = GridDetails.Rows(RowIndex).FindControl("DropEndTime") If Not (IsNothing(ComboStart) Or IsNothing(ComboEnd)) Then ComboStart.Items.Clear() ComboEnd.Items.Clear() ' -------------------------------------------- ...