Can't unshare workbook

If I go to Tools > Share Workbook and untick the box to STOP sharing it.

I get the confirmation message and Yes to continue unsharing it.

However, the workbook remains [Shared].

I am doing this on the same machine which I used to make the workbook shared in the first place. Same version of Excel.

If I go to Tools > Protection, the only option is "Protect Shared Workbook" ... so it is NOT already protected. I've tried protecting, unprotecting and then unsharing with no luck.

Anyone got any ideas?

It's driving me mad and by turning ON the sharing, I've broken the VBA code which runs in the background. :-(