Plex libraries not recognizing folders on DX517 expansion unit

My Plex libraries are not recognizing shared folders and subfolders on a DX517 exapnsion unit.
I run PMS on a Synology DS1522+. I recently added a Synology DX517 expansion unit. Both PMS and Synology are on the latest version.
I added a second storage pool and volume on the expansion unit just like on the DS1522+. I also added a new shared folder and gave the Plex user read/write permissions. I then created a new subfolder for movie files under the new shared folder on the expansion unit. However, when I browse to add additional library folders on the expansion unit to my existing Plex libraries, I cannot. The Add Folder option only sees the new shared folder, but does not see the subfolders. And, even though it sees the new shared folder, if I add media files directly to it, the Plex library does not see the added media files.
What is the fix for adding folders to my existing Plex libraries so that the Add Folders option sees and recognizes the new subfolders on the expansion unit?