Instead of .mp4 you typed ,mp4 or .m4p?
Or you forgot to put the extension on entirely?
Filter: Everything
Sort by Size
!video: !audio: !pic:
Scan the topmost (biggest) files.
See some big file, misnamed, that you've forgotten about, that you haven't played - because you didn't realize it was there, because your normal video: search didn't find it, because it was misnamed...
And you can filter further as you go along:
!zip: !doc: !exe: !folder: !.iso
Amazing what might turn up
.