Running out of ideas.

If it's just one Excel file that's affected, it might be worthwhile ensuring you don't have sharing permissions for the file (Review>Share Workbook, then uncheck 'Allow changes by more than one user at the same time). Another issue that might affect a single file is sheet grouping. This 'group' mode can be identified by '[GROUP]' next to the filename in the title bar (eg <filename.xlsx> [GROUP]). To remove this, I Right-click on the active sheet at the bottom of the screen and choose 'Ungroup sheets'.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]