Official comment
Hey Midgy,
What this means is you imported the songs, moved the files, imported them again, and then tried to relocate them... but since they were already in the collection, having been added that second time, they're not actually missing, but rather you have two entries for it.
First, sort by song name. Find the song that is missing and compare it to its twin which is not missing; what is the import date for that second one? Is it the same for a large batch, if not all, of the 400 missing songs? If so, sort by date, find all of those new entries, delete them, then relocate and the old entries will be matched with the new file locations.