If you're posting files that are muxed with MKVMerge, they won't be playable on many players if you're using the default settings of MKVMerge 4.1+. The workaround is to use MKVMerge 4.0 or, in newer versions, turn compression OFF on video and audio tracks. To do this:

1. Select each track (video, audio, and subtitles)
2. Select the Extra Options tab
3. Set the Compression option to "none"
4. Start muxing
I got this from another site and it work for me. Just run your file through mkvmerge with compression OFF.