Albums on NAS not found each time core reopened

NAS (Synology). Core running on MacBook Air.

C13,500 tracks on NAS which the core finds eventually each time the Air is booted. But on start up, there are only c8,000 tracks available and the Core has to go through a lengthy process of ‘refining’ the ‘lost’ tracks on the NAS.

Why doesn’t Core keep full list of tracks on NAS?