Libraries not auto scanning on new media being added

Synology is running on a Linux based system.
If you already have a lot of media folders and subfolders in your library… the system might have come to its limit of folders it can monitor (default limit is approx. 8,100 folders).
You can change that using this post in the Linux tips & tricks:

If you don’t want to increase the number of directories (e.g. potential impact on overall system performance)… you can also exclude music libraries from automatic updates (this usually results in a significant reduction in folders to be monitored). See Settings > [Server Name] > Library > Include music libraries in automatic updates

Also… and just to be on the safe side: have you recently double-checked if you have enabled the related options in Settings > [Server Name] > Library (e.g. Scan my library automatically or Scan my library periodically).