Video Converter Youtube MKV videos won't convert, WEBM will

[llandyw]

Why is it that WEBM files convert just fine from Youtube, but MKV files do not?

WEBM is MKV just with the "matroska" replaced with "webm".

Yet, it always says that there's an error in the file. The error is:

"Failure opening the selected file, please check and make sure it's playable"

Yet, other software, using ffmpeg, has no trouble with the vast majority of these files.
It's extremely slow compared to the conversion of WEBM files using DVDFab converter (at least a 10 to 1 difference as ffmpeg doesn't use the GPU).
So, what's the deal? Is it just not being accepted because you want the downloader to be used, or have they been excluded for another reason?

Andy

[signals]

Please supply the log files and MediaInfo text output for one of these that works and one that doesn’t. No need to mention other software by name or to assume sinister motives on the part of DVDFab, which Is ridiculous and borders on paranoia.