I have been poking around the Plex forums trying to find a way to fix this and have not had any success. Re-ordering the agents and performing the "plex Dance" on the server do not seem to work. I can manually download all the posters I want, but that seems kinda time intensive with a library of over 1000 items. I don't think it is a permissions issue as I can manually edit and assign posters. Subtitles seem to be coming down pretty regularly. I haven't checked every TV episode and movie...
Plex movie/show posters no longer updating on Plex Plugin
Plex movie/show posters no longer updating on Plex Plugin