Spitballing (as I don't have much info), but it seems that some of your media is able to be played directly and some needs transcoding. There should be logs under "Dashboard/Logs/[playtime].log" in your server admin. that hopfully provides more information. With the info provided it _seems_ that there's an error with your ffmpeg install (this is a guess) but either looking through the logs to halp figure out what's wrong with your transcoding, or externally transcoding everything to something that's univerally supported (x264 through handbrake usually) could help.
