ffmpeg WebM output videos freezes after a few seconds

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http://unix.stackexchange.com – I have ffmpeg set up on my computer, and tried to convert some H.264 encoded .mov videos to WebM format. The command is: ffmpeg -i [inputvideo].mpv -sameq [outputvideo].webm However, for many (but somehow not all?) of the output videos, they would freeze after playing for a few seconds, but the sound would continue. What does this mean? And how can I troubleshoot this? Thanks! (HowTos)