F1 key not working on Mac mini — but it's not a mechanical issue.

For years I've been using the F1 key in Quark Express to apply a style sheet when working on documents.

The other day it stopped working. I changed keyboards. Still doesn't work. All other Function keys work btw.So I brough up the visual Keyboard viewer app and pushed F1 there. STILL no F1 function. On the viewer the F1 key is highlighed when pushed - but it doesn't do an audible 'click' like pushing any other key.

It doesn't' seem to be hardware. It happens across all apps and in all cases. Pushing the F1 does nothing. I've restarted the computer. I've run Cache Cleaner. changed keyboards. Nothing works.

Any ideas?