Music artist name sorting

Hi there, I have an odd problem. When I sort my music collection with the artist name, Plex does it correctly most of the time, but in some cases it uses the surname, not the first name. For example, Hans Zimmer album (Dark Knight) is in Z, not H. If I delete Zimmer from the Artist Name, it goes to H. Same with Andrew Lloyd Webber. It is in W, if I remove Webber it goes to A (not L) !!!
I checked all the mp4 id tags, with a 3rd party program and there seems to be no problem.
Any ideas?

Thanks