What photonboy said. Except for VLC, changing video players won't help.
All video players just call Windows to handle the video decoding for them. If Windows doesn't have the proper codec installed, it won't be able to decode the video, or will have trouble doing so. So install a good codec library like K-Lite. VLC is unique in that it uses its own built-in codecs, and doesn't rely on Windows. Kredo dat 41 s kryakom. So it's a good video player to keep on the side to help you diagnose problems (or just use it as your standard video player if you don't want to bother with installing codecs).
Also, MKV is just a container. The actual video file can be any number of formats - MPEG4, Xvid (MPEG4 + ASP), H.264, or now H.265. So just because one mkv video players doesn't mean all mkv videos will play, and vice versa.
Other popular 'video formats' are also just containers as well - AVI, MP4, WMV, etc. They're just containers which glue a video file, an audio file, subtitle file, chapter index, etc. Together into a single file. Installing a codec library adds support for MPEG4, Xvid, H.264, etc., especially for non-standard or new or non-standard implementations like Hi10 (10-bit encoding).
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