Results in Test, but none in Virtual Column

The following expression generates correct results in Test, however the virtual column displays 0 on all records having a [Closed Date] value:
ISNOTBLANK([Closed Date]),
HOUR( [Closed Date] - [CreatedDate])/24,
Thanks in advance.

I’m not sure why your test results and actual performance would differ. I’m responding to this in order to put it at the top of the list in the hope that someone else, who is able to help, will see it as a result.