What stops my screensaver????????

Friends
never seen this one before. On a fully updated, fine-running Windows ME
system, with Active Desktop "On" and a colorful personal picture as
background, the screensaver just died a few days ago, and even though it is
'on' does not kick in. I can force it by doing the "Preview" function under
Display Properties, but will not go on its own.
I'm out of ideas.
We ARE using a scheme.... one of the sporty ones with a lot of color.

Please advise
TW
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Consulting for growing local businesses