When Cell Updated Another Cell To Be Changed

I have a Schedule Grid that I've created. When a cell gets updated, I'd like for another cell to be changed (as to trigger some conditional formats that I have going on). I figured it'd be rather easy, and it was... sort of.

This is the code that I am using:


Private Sub Worksheet_Change(ByVal Target As Range)

If Target.Column < 4 Or Target.Column > 10 Then Exit Sub
If Target.Row < 4 Or Target.Row > 422 Then Exit Sub

Target.Offset(0, 9).Value = 1

End Sub
The code worked perfectly, up until a random moment. Not sure why, but now, the code only works when the cell is updated by a manual entry or deletion. If I use a validation list (which is the primary form of entry), it runs every line of code EXCEPT the Target.Offset(0,9).Value=1.

Here's the strange part. I took it home, and it works fine there. It used to work fine on my computer at work, now it doesn't. Nor does it work on the computer that I need to present this on tomorrow.

I've tried entering in a MsgBox() event both before and after the Target.Offset line. The first message box appears, so I know the code is passing through the restriction lines. But the second message box is not appearing. The code for whatever reason exits the sub at the Target.Offset.