Since you have tried about everything, give the connection process another shot to get the internal Sonos logs updated and send in a Diagnostic. Then contact Sonos with the Diagnostic number to have them look at the data you can’t see.

SONOS is rather impatient. If there are delays on the network, the players will give up. Check the PING times to the NAS. Another dimension seems to be the size of the library. The indexing process is carried out in a player. When complete, the updated index is copied to all of the players. You don’t have much control of which player executes the index. If the indexer is struggling with its connectivity or a very large library, there could be a timeout. 

It’s hard for me to describe “large library”. I recently cooked up a library of 60000 tracks and this was easily indexed and I could throw all of these tracks into the Queue. Constructing a Playlist of 10000 tracks was a non event and this was only one of a few existing playlists. I’ve seen users struggle with a few thousand tracks and one user struggled with 800 tracks. I think that it depends on the structure of the metadata. My cooked up tracks have very short metadata. I’ve seen some rippers throw the whole first stanza of an opera into the file names. This uses extra space and processing time. My file names are very short, such as TRK01.flac, etc. I don’t recall the exact details, but long meta data must occupy multiple slots in the index. Obviously, this ties up more memory in the player running the indexer, but it also slows running time -- risking a timeout.

A quick workaround for excessive metadata is to break your library into a few shares, rather than one large share. Up to 16 shares is supported. Rather than \\drive\music use \\drive\music\classical, \\drive\music\pop, \\drive\music\jazz., etc. This will reduce the indexer memory footprint in the player and reduce running time for each segment. As far as the user interface is concerned, only the Folders view is impacted.

---

Pure speculation: I can imagine if the player running the indexer has a large Queue, the indexer could be memory constrained. Try emptying the Queue in the “Associated Product”. You can find the “Associated Product” in “About My Sonos System...”.

I’ve clicked the tick on this item to find out what it does and it’s marked your response as the answer without me actually confirming. Whoops!

Anyhow, you’ve given me something to go and gun down this afternoon. The library is at about 8k tracks.  Over the years iTunes and other tools have been steadily adding metadata to tracks it recognises including artwork and track data. We also have many of our own recordings and language material piling in (though the metadata on those should be minimal) and the content is several 10’s of gigs.

There are 2 devices with marginal Wifi connectivity.  Fine for streaming but the connectivity shows some issues when using Alexa or Google voice commands. If one of those units is being selected for the library update I will isolate that as the issue by keeping only the wired and full signal strength devices connected during the process.

Thank you very much for responding.  I’ll post again tomorrow.

File size is not an issue.

Hi, I’ve just stumbled on this thread while I’ve been searching for a solution to a problem I encountered.

I’ve been running my Sonos for about 3 years with a fairly static (I.e. not growing) music library of about 11000 tracks.

yesterday I started adding artwork to those albums which didn’t have it and all was well till I noticed that one track on an album I’d updated with artwork was not showing it..only one track.

Thinking it was just a glitch I tried to add artwork to another album and then things started to fall apart. I couldn’t play back anything and then the library disappeared completely from the Sonos app.

The library is on a qnap which I could still ping and access from my tablet and play music through another app. So no problem with the device or network etc.

I tried to add the library again which seemed to start ok but after about 10 mins it gave up and came up with message “device is no longer online ..” pretty much as per the Op on this thread.

after a couple of unsuccessful attempts to add the library I wondered about the index thing and decided to take cover art off of about 200 tracks. Lo and behold, library is now added and working fine so far as I can see.

so it seems that the artwork is the cause. Which is a real pity cos I’ve only got artwork on about 40% of tracks. 
 

now a question. Does Sonos pick up artwork which exists within the folder which contains the album and use that to display at the folder level , but without adding it to each track? That would make a big difference (esp when you consider some albums may have up to 20 tracks)

hope my comments are useful in adding to the topic and apologies if this issue has already been “resolved” in other threads

@Mcgoverg

Your story there has a potential parallel for me to follow. I have iTunes, set to add artwork, which may have hit a tipping point at some time.

And to report back on my previous comment, I don’t think I have an issue with a unit with poor wifi connection failing to connect to the QNAP yet taking control of library update.  I switched all units off leaving one wired and one wifi unit only 4 steps line-of-sight from my Airport wifi router but had exactly the same symptoms whether trying to direct the library update by NAS name or IP address.

A further update, I decided to remove artwork from all but the 1st track of each album. So in viewing the collection either by artist or by album it is displayed with the cover art, but without the overhead of applying it to every track. 
 

Not sure if this may assist here. Sonos support a resolution of 1024 x 1024 pixel for their embedded cover art as mentioned here…

https://support.sonos.com/en-us/article/missing-music-library-album-art-in-the-sonos-app

I personally use 600 x 600 px and that resolution seems to work okay with my own NAS library (approx. 25,000 tracks). It all indexes and displays okay in the current version of the Sonos App (using SMBv2 min. and SMBv3 max.)

If you add a “folder.jpg” or “folder.gif” file to the folder containing the track files, there is no need to modify any tracks. One image file will service all of the tracks in the folder. This is a quick way to add or edit artwork. Note that some other players might insist that artwork is embedded.

Yes, just to add, as @buzz mentions, I also hold an extracted album art 600 x 600px "folder.jpg" image in each album folder in the local library, aswell as the art being embedded in each track. I perhaps should have mentioned that.

The tag editor software ‘MP3Tag’ makes it reasonably easy to batch-add, or extract, such album art images.