I found, that i could not convert M4V videos.
The reason is, that eZ (4.4) does not know the .m4v extension and does not return the valid MIME-type for it (video/x-m4v). As a result, ezvideoflv refused to convert it, because it does not identify it as a video.
create a mime.ini.append.php in /settings/override and add
Then clear your INI cache and convert the files again.
php runcronjobs.php flv
php runcronjobs.php flv -all
for re-converting the files.
If that does not help, upload your M4V files again.
The other solution is to rename the M4V files to .mp4 before uploading.