Grid Control with vertical scroll bar does not see rows that are hidden

Hi all,

I have a grid control sized on a form. If the dataset has 100 rows, the control has small height, and shows only 10 items, the rest of the items need to be scrolled to view. 

I have a button that I want to loop through all the rows, however, when I start the loop, it only sees the 10 items and exceptions on the rest of the columns, if I resize the grid and make the height bigger to fit all the items, it will see those rows in the code.

How can read all the rows regardless how small the hieght is.

   Dim payment As PaymentDataItem
        Dim ticketIds() As Integer
        Dim referenceNos() As String
        Dim i As Integer = 0




        For Each payment In CType(uxPaymentsTable.ItemsSource, PaymentDataItemList)
            Dim rowCheckbox As New CheckBox
            rowCheckbox = CType(uxPaymentsTable.Columns(4).GetCellContent(payment), CheckBox)

            If rowCheckbox.IsChecked Then
                i += 1
            End If
        Next

Thanks

Al