I just ran into this again and I had to spend more time and effort than I or someone else probably wants to find a good answer.
The problem is that too many people answering these questions only suggest update-grub
(see 11, 22).
The best answer I could find so far is this onethis one on GRUB does not detect WindowsGRUB does not detect Windows. This should be a extended to signpost answer covering typical cases why os-prober
is unable to find any installed version of Windows (or any other operating system).
Braiam's approach was to hope that the system will heal itself. From my point of view this hasn't shown any progress since then, it has failed and needs our support to find a good answer to this question.