Cover art not updating for MP4 files in Windows Explorer

Hello everyone,

Maybe this is a known issue, but I searched around and didn't find a solution. The only solution I found involves VLC Player, but my issue is with Windows Explorer thumbnails, not VLC.

I'm trying to update the cover art of a MP4 video with Mp3Tag, but it doesn't work. It keeps the video original cover art, which I do not want. I'm using Media Preview for thumbnail generation with "Cover Art" option enabled. My cover arts are showing correctly for MP4 videos I converted from other formats, but I'm not being able to change arts for videos I already downloaded as MP4. I could reconvert them, but that is too much trouble.

Anyone knows what may be happening?
Oh, I'm in Windows 7 and usually use Media Player Classic for reproducing the videos.

Thank you all!

EDIT: I should have written "some MP4 files" in title, because I can change cover art for some original MP4 files as well, but for some others I can't.

It is not quite clear whether you cannot change the cover in Mp3tag
(e.g.: load file - load new cover - old cover still displayed in Mp3tag) or that the windows explorer does not show the right thumbnail.
If the latter is the case, then enable the display of hidden files and delete already existing thumbs.db files and folder.jpg files.

Also, some Nero Mp4s have been writen with incompatible tags, you would have to un-nero them.