Hello, as asked by Bastien, here is a thread about my problem where all the covers I have submitted don't reach a 100% in the completeness bar.
Attached is an image of my last submission, the Cuban Pete version of Jim Carrey performed in The Mask and released in the movie promotional singles.
As you can see, I have provided two sources for my submission, the All Music page of the song and the IMDb soundtrack page of the movie (that one is a complement as it gives the information on the actress that also sings the song).
Despite including that information, the box remains with a red mark as if it was empty, and that's why I can never reach a 100% mark in the completeness bar.
I hope this information is useful, if I can provide something else let me know.