I might have misread your OP. If some photos can’t be displayed at all, then a Synology NAS running PMS can suffer from inadvertent perms / ownership issues preventing full access to your media. That FAQ is critical if you haven’t seen it lately. The pinned topics in the same NAS & Devices - Plex Forum are great.
If the issue persists after doing chmod and chown then you can restart the Syno and reproduce the issue, screenshot what you see, and grab server logs with Settings → Troubleshooting → Download Logs
Note: if only some thumbs are missing
You can use the Plex Media Scanner on the command line to generate thumbs.
You can probably use Tatulli or Webtools-ng to generate thumbnails too, but those are a lot of Swiss army knife if you just have missing thumbs that can be done with:
“/path/to/Plex Media Scanner” --generate --item 2356
