Please post the FFmpeg information on the AVI file that does not display subtitles. To do this, right-click on the file in Mezzmo and click 'Get FFmpeg Information'. Then, post the FFmpeg information on an AVI file that does display subtitles (you previously mentioned that you do have subtitles working for some AVI files). We should see the difference in the two AVI files and that should provide a good hint as to why you are having problems with a particular AVI file.
Bookmarks