"Private Sub Worksheet_Deactivate()" doesn't trigger

I'm using Excel 2003 on Windows 10.
I'm trying to use:-

"Private Sub Worksheet_Deactivate()"
"Userform1.Hide"
"End Sub"......

....placed in the code area of the worksheet,
to dismiss a Userform from that Worksheet,
when I leave it by selecting another Worksheet's tab.
It doesn't trigger.
If I run the code manually, it works.
There are no "Application.EnableEvents=true or false" lines in the code.

I'm curious to know what can be stopping the code from triggering.

If I write a simple, separate, stand alone programme to do the same thing, it works fine.
So presumably there's so presumably, there's something in my code causing the problem,
but I don't know what to look for.

In the wider world of Excel, there must be some known quirks that can cause this behaviour?
Help please

I did have a module in the workbook with conditional formatting, but it's removal had no effect - still no triggering

Hi MikeRickson
I tried your suggestion of using a Workbook level SheetActivate event, but again there is no trigger.
I did try a simple programme with the basics of what I'm trying to do and that worked fine.
If I MANUALLY step through the code I'm trying to trigger in the problem Workbook , it works fine.
So it seems as if there's some setting in my code that's inhibiting the trigger event.
Still baffled !