Well, I saw a thread that was locked, regarding files that had used the internet search and download feature for cover art, leading to files no longer being recognized and able to seed in uTorrent, which is what happened to me.
Myself, though, i assumed it was my editing of id3 tags. Though, of course, when i imported all of these new files into MJ the first time, I didn't keep track of which ones i downloaded cover art for, and which ones i used the get info from filename feature for. But shortly thereafter, i noticed that some of my formerly seeded files were no longer seeding. And even relocating the individual files one by one did nothing. The file itself has been modified in some way that prevents it from being seeded any more.