is there a problem with episode images

[bjjones 22]

They seemed to have stopped working on the latest series I've added. Series and usually even seasons get art but the individual episodes seemed to have quit. On a later one 1/2 the episodes have an image, rest are just the image of the series. I've also got a cover as an attachment in the .mkv which I thought would get used if necessary as a backup to online but I can't see a setting to use those so I'm not sure what/when/why those are even useful?

On DSM 7.1.1 Emby 4.8.11 with TVDB and MovieDB plugins installed - thanks