Linkage issue: Find Missing Files not working?

I just did a test relocating a linked .blend and then from the File > External Data menu chose Find Missing Files
…but even though the .blend it depends on was in there it didn’t find anything! What gives?

If I can’t get this sorted I may not be able to relocate a project I’m working on.
Any help appreciated.

Make sure the names match up as well, you might also want to play around with Make Paths Relative under External Data and such.

.blend file names are the same, just in a different directory. Relative file paths are the only kind I use.
Thanks for suggesting, but it’s neither of those.