I hope this isn't a common problem, but I haven't been able to find any posts that seem to apply.
I'm having an issue scanning a TV show with a lot of seasons (Doctor Who). Something is choking as soon as it hits the 19th season: the additional episodes/seasons get added to the library, but it doesn't add in any episode metadata or cover image information for any of them.
It doesn't appear to be an issue with the files themselves. The first few times, I scanned the entire batch at once; it scanned the seasons out-of-order and consistently ended up with a block of blank seasons in the middle starting at season 5. I've pulled the whole show out and completely uninstalled and cleared my installation, and then tried adding the seasons one-by-one, and it consistently stops adding new metadata when the total number of seasons hits 20 (counting season 0) regardless of which particular season tips it over that number (for instance, if I skip seasons 16, 17, and 18 then it stops adding metadata on season 22).
I've found a few errors/warnings in the logs when it hits this point, but they aren't telling me much. The most relevant appears to be an exception in 'PMS Plugin Logs/com.plexapp.system.log'.