I have a strange thing happening on a single file. When I open the file under it's original name, the highlighting is lost (":syntax on" does nothing). But if I cp that file with another name and open it, the syntax highlighting returns. If I mv that copy back to the original file name, the highlighting is gone again.
I can't show you images (as my rep is not high enough), but I assure you if I could, you could clearly see that file a.js has no highlighting while a_copy.js does.