Plex API does not allow to sync all the album data with efficient queries.
To be able to gather all the additional data it is necessary to do one request per album, greatly increasing the sync duration. For that reason by default those are not synced.
You can easily enable the full sync by editing your media provider settings (In Settings / Manage media providers) and checking the Full sync option. (Do not forget to press the update button then force a sync in the filter menu).