@bradbart948 - I too am experiencing this since the last few PMS versions, and have created a thread detailing my findings so far…
However for me, although I have found this to be new behavior, I have narrowed it down to having special characters in the folder name, such as (year). If I remove the (year) from the folder name, the chapter thumbnails are generated.
I too am a little frustrated by this, and by the sheer amount of oddities that have been introduced in the last few months!
For me, I’m just gonna keep bumping my posts until someone takes notice… Or, simply ask any a Plex employee that I can find on the Forums the very specific Question… “Are you looking into this???”
