MKV files and iPad

Hello,
my trouble is with iPad.
I download files as matrioska (ext .mkv) on my computer (under windows 10)
Then I put this files on usb Sandisk key and read them on my iPad using the Sandisk application.
Sometimes there is no problem, but sometimes I cannot read them:
it looks like it is a codec problem; could you help me solving it?

Many thanks
Have a nice day

Right-click on the file on your computer and choose MediaInfo. That will show you details about the file and what kind of audio and video formats it contains.

H.264/AVC video is what is most compatible with your iPad.

For files that currently do not play you could try the VLC Player app for IOS. Otherwise you will have to convert it to a compatible formats with for example Handbrake.
https://handbrake.fr/