Universal Media Server | Forum

Logs will be needed to see what is going on. Follow the link above in red and post the logs here.

It will be your config file not matching with the capabilities of your TV. MKV files are just containers and can have different video formats, which is why some work and some don't. It is likley the ones that are not working are encoded in h265 (HEVC) which is a fairly new and takes more processing power to decode so a lot of players can't play them.
As the samsung-NotCD.conf just a generic config and it matches MKV, UMS will assume the TV can play anything with an extension MKV.

try adding under Supported video formats, that should only transcode anything that is not using the old codec (h264) in a MKV file.

had a quick look though your logs. I was expecting to see some more info about the format that files were in cut couldn't see that, maybe because it is just matching filetype and not the codec, I'm no expert though so maybe one of the devs is better place to help.
the info on the 2 files wasn't much help for me either but one thing I did notice is that the first one is in SD and the second is in 720 HD. I was hoping to see something like Video: MPEG4 Video (H264) or Video: HVC1.

Ideally your TV should be matching with Samsung-CD.conf and not the Samsung-NotCD.conf, however the config does say

Note: if the device name "[TV]PS51D6900" has been edited by the user via the TV menu it will likely be misidentified as E and later series

you can override this with your own config though. I was able to do one for my F series as i could find what media is supported in the manual, and have used that to hopefully make one that will work on yours but i have no idea what yours supports so doubt it is 100% right.