MKV video cannot display thumbnail

dear:
I'm sorry for my poor English
All videos downloaded through the software, 4k 1080p 720p, as long as they are in MKV format do not have thumbnails, and the computer cannot recognize them video frames height and width.
I tried to install a lot of decoding software, but none showed thumbnails, such as:
"ADVANCEDCodecs","MediaPreview" and many other.Video too many, I do not want to re-download, how to display thumbnail?

It is not the fault of my computer. Other MKV videos in my computer that are not downloaded by your software can display thumbnails