Some times create thumbnail and some times not is due to bad installation of ffmpeg/ffmpeg-php. You may need to reinstall latest version of both ffmpeg and ffmpeg-php. If you check apache error log when this happens you will see some ffmpeg errors when thumbnail is not created.

ffmpeg test script availabe at

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Checke the following for fixing

Convert Error or converted video is less than 1 kb in size, try upload a big video. If that do not work, verify convert command works on the server.
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