Worksheet change failing to initiate

Hi

Have code where I want an action to happen based on a cell value changing.

The code below works on other cells and sucessfully completes the action, however for some reason this specific cell range fails?

I cant give an example as it only happens on the actual worksheet( so cant share)

What elements should i look out for that make this fail ??

Private Sub Worksheet_Change(ByVal Target As Range)

If Target.Address = "$g$12" Then

Range("G20").Select
ActiveCell.Value = "=NOW()"
ActiveCell.Offset(0, 1).Value = Windows.Application.UserName


End If

End Sub