Finding *.mkv and *.mp4 works
find /home6/movies/ -name '*.mp4' -o -name '*.mkv'
but moving them for some reason partially fails and moves only mkv files
find /home6/movies/ -name '*.mp4' -o -name '*.mkv' -exec mv {} /home6/archive/ \;
Am I using an incorrect find switch "-o" for this task?