Music videos not showing up in Music app (Mac)

Dear Mac users,

I need your assistance!

Long time iTunes/Music user here.

I have a huge external library, which contains music and music videos. None of the videos were bought on the Apple store and I' not an Apple Music subscriber: most of my videos are musical programs I have recorded on TV, home videos, commercial DVDs converted to m4v, etc.

Right now, I'm importing a lot of new videos. It's going smoothly with most of them. BUT: others are a mistery to me.

Here is my process:

• I select a video and import it to Music (usually using drag & drop, but I also tried using File > Import)
• The file IS copied. I'm 100% sure of that, because I double check the path (Media > Music > Artist > Album > Video)
• And then the video is nowhere to be found in the Music app interface :-(

I have used both the search engine on the left column and the search engine on the right (after selecting the "Music videos" section in the left column ) --> the video is still invisible.

I have tried changing the extension from m4v to mp4 --> fail.

(I have changed the name just by typing - should I re encode the video? I have hundreds of them...sigh)

I don't know what to do!

Any help or clue would be greatly appreciated!

Thanks in advance.