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Perhaps they're locked? FM has so many keyboard shortcuts, it's just about possible to do this by mistake … Take a look at the menus, and see whether more commands than usual are greyed out.

To unlock a document, use the sequence Esc, Shift + f, l, k – the third character is lower-case l, not figure 1
You can also select multiple .fm files in the book panel before using the unlock shortcut.

Anecdotally, I usually hit this problem in the other direction: the FM shortcut for "paste special" is the "set read-only" shortcut for my usual text editor.