FrameMaker 2020 sidehead mystery

[StudioSm]

I have some documents with a sidehead page layout. When I change some of the paragraph styles which have had a setting to "In Column" to "Across All Columns and Sideheads", I get this message.

But the page layout HAS room for side heads and FrameMaker accepts the changes and the paragraph style works as intended across sideheads and columns. But apparently FrameMaker is still dissatisfied. But with what?....

Has anyone experienced this? Am I overlooking something obvious?...

[Barb Binder]

Are you using Flow A? Are the sideheads defined on the master pages for Flow A, and do the body pages have overrides? This is a tricky one. I'm happy to look at the file if you feel comfortable sharing it. 

~Barb