InDesign Spellchecker stops working

I’ve noticed that the spellchecker keeps stopping.
As in, if I check spelling, it says “done/Complete” i.e. no spelling issues.
But if I copy paste that same content into a brand new document – then try Spellcheck again, it picks up errors.

It seems to depend on when the file was originally saved.

But that's not great, as I'm constantly changing or adding text to client files for years on end.

So I need the spellchecker to be reliable and not stop working just because the original creation date of a file is old.

Any ideas?