Most probably those files are being transcoded and the speed of transcoding is not fast enough for smooth playback. Did you adjust the network type in device properties dialog by any chance? If you have, I'd recommend changing it back to the default, because any changes in there can lead to transcoding even if the files can be played over your connection.

As far as I know, Samsung TVs don't support embedded subtitles and that's why they're not coming up. The next major update of Mezzmo will include support to rip out and re-burn the subtitles onto the video (or serve as external subtitles), so that will enable subtitles for those files.