Problem with public global declared variables

I am trying to figure out what I am doing wrong with global/public declarations. In the code I am writing, I have the following code in the “ThisWorkbook”

Public HemiRes As Double

Private Sub Workbook_SheetActivate(ByVal Sh As Object)
If Sh.Name = "Data" Then
ActiveWorkbook.Sheets("Data").Range("A2").Value = HemiRes
End If
End Sub

Then in another sheet I have am using the command to transfer the value:

Private Sub CommandButton1_Click()
HemiRes = Worksheets("Hemi").HemiRad.Value
End Sub

The value of HemiRes is always 0, no matter what I seem to do. Is there something I an not correctly understanding about the way I can save values to a public declared variable from any sheet?

Thanks