For some reason, 3 of this 4 videos won't have a preview image. The default program to open them is Quicktime, so it's kinda weird. I can still preview them with Quicklook, but it bothers me that they appear as a white paper....
Because QuickTime doesn't support .mkv files natively.
I guess it's to due with the Perian plugin. Try QuickLooking an .mkv file, and scrub. Doesn't work. Maybe a bug with Perian, or just the limitations, who knows.