I just discovered if i use rebuild thumbnail I get an image
Which means that there were images next to the video files for Epsiodes all along, and that the Thumbnail files may have been corrupted at some stage.
From memory you connect to your Library from two different PCs, at different times. i.e. From your Workstation for maintenance and from your HTPC for playback. Thumbnail files are stored on and used by the PC which connects to a library, which means that your Workstation and HTPC have two separate sets of thumbnail files. I think I have mentioned previously that this may cause an issue. After every time you do maintenance on your shared library using your Workstation, you probably need to run the "Tools > Advanced Tools > Build missing thumbnails" function to ensure the thumbnail files on the HTPC are updated. Or maybe run the "Tools > Cover Art > Rebuild thumbnail" function on any programs that you think have the wrong or missing thumbnail.
So basically I think the topic of this thread is a consequence of the way you share one library and access it directly from two PCs, assuming that is still the way you do maintenance. (Yes, I understand why you do it that way. I wish all maintenance could be done from a Client myself.)
Having said that, of your two examples;
Australian programs like 800 Words tend to get less love on TheTVDB than US programs. It only has Episode images for Season 2 and a few episodes of Season 3 on TheTVDB. See
https://www.thetvdb.com/series/800-words/seasons/allMadam Secretary, on the other hand, has Episode images for all but episodes 2 to 18 in Season 2. See
https://www.thetvdb.com/series/madam-secretary/seasons/allYou can always check TheTVDB directly, as I did using the above links, to see which episodes are missing Cover Art. Maybe you could pick some good images and upload them for the episodes that don't have them? You just need to meet the requirements of TheTVDB, which can be found somewhere in their forum I think. Or ask in the forum if unsure how to proceed. Or perhaps ErikN can point you to the location of the rules, and the procedure for uploading images?