Excel 2007 : Excel 2007 BeforeSave Cancel = true not working

Here is the code. Pop-up is displayed, but workbook is getting saved. :-(

Private Sub Workbook_BeforeSave(ByVal SaveAsUI As Boolean, Cancel As Boolean)


Dim ws As Worksheet

Application.ScreenUpdating = False
For Each ws In ActiveWorkbook.Worksheets
If ws.Name = "ResourceList" Then
Else
If Trim(ThisWorkbook.Worksheets(ws.Name).Range("B1")) = "" Then
MsgBox ("Project ID is required for " & ws.Name)
Cancel = True
Application.ScreenUpdating = True
Exit Sub
Else
If Trim(ThisWorkbook.Worksheets(ws.Name).Range("D1")) = "" Then
MsgBox ("Project Name is required for " & ws.Name)
Cancel = True
Application.ScreenUpdating = True
Exit Sub
Else
If Trim(ThisWorkbook.Worksheets(ws.Name).Range("F1")) = "" Then
MsgBox ("PM Name is required for " & ws.Name)
Cancel = True
Application.ScreenUpdating = True
Exit Sub
End If
End If

End If
End If
Next ws
End Sub