Metafiles / BR Cover for MKV files

Hi,
I am currently doing some tests with Serviio and until now, I really like this programm and appreciate your work
To make life easier, let's call BluRays "BR"
I am only struggeling with one part: I've activated the automatic Meta - Information Research (not sure if it is called exactly this way in english).
My Files are all within one folder, I did not seperate them in any way.
The filename for common BRs is just the name of the movie, like <name>.mkv
The filename for 3D BRs is the name of the movie and a 3D prefix, like 3D - <name>.mkv
The Meta Information Research works pretty fine for most of my BR and 3D BRs, round about 80%.
But some movies do not get a cover when I look on my TV for any reason.
I already tried to add the year to the file name like 3D - <name> (2015).mkv but this didn't help at all.
My next try was to download a JPG file, call it dvdcover.jpg
Then I used MKVtoolnix, went to "Attachments", right clicked in the lower box and attached this JPG file.
Then I muxed it
(Love this word...)
But unfortunately this does not change anything, the movie still got no cover on my TV.
What went wrong, how can I handle this?
I am currently doing some tests with Serviio and until now, I really like this programm and appreciate your work

To make life easier, let's call BluRays "BR"

I am only struggeling with one part: I've activated the automatic Meta - Information Research (not sure if it is called exactly this way in english).
My Files are all within one folder, I did not seperate them in any way.
The filename for common BRs is just the name of the movie, like <name>.mkv
The filename for 3D BRs is the name of the movie and a 3D prefix, like 3D - <name>.mkv
The Meta Information Research works pretty fine for most of my BR and 3D BRs, round about 80%.
But some movies do not get a cover when I look on my TV for any reason.
I already tried to add the year to the file name like 3D - <name> (2015).mkv but this didn't help at all.
My next try was to download a JPG file, call it dvdcover.jpg
Then I used MKVtoolnix, went to "Attachments", right clicked in the lower box and attached this JPG file.
Then I muxed it

But unfortunately this does not change anything, the movie still got no cover on my TV.
What went wrong, how can I handle this?