Not much information to go on, but I’ll hazard either a duplicate IP address issue, or simple wifi interference causing the indexing to stop. You may want to submit a system diagnostic within 10 minutes of experiencing this problem, and call Sonos Support to discuss it.

There may be information included in the diagnostic that will help Sonos pinpoint the issue and help you find a solution.

When you speak directly to the phone folks, they have tools at their disposal that will allow them to give you advice specific to your Sonos system and network.

thanks, Airgetlam, even when I re-sync update multiple times it's still the same tracks that are missing each time, if it was Wi-Fi drop out, I would have thought it would correct itself at some point.

Using the “folders view” in Music Library do you see all the tracks and do they play correctly?

No, can’t see all the tracks, they’re missing from Sonos library altogether 

Roughly how many tracks do you have in your library?

26,000

Ok, you need to run an index again and immediately afterwards submit a diagnostic, then ring Sonos for them to look at the logs. 

I would maybe check the tag metadata on the missing tracks and just see if there any anomalies like special characters, that might be causing them to fail to index - here are some of the limitations too:

• Album Art should not exceed 1024 x 1024px

Field Name Character Limits…

• Artists - 76

• Album - 92

• Track/File Name - 100

• Genre - 22

Maybe use something like MP3Tag (tag editor) software to check things - it might just be a special character causing an issue (perhaps ?) If not resolved then after re-index failure. It maybe worthwhile submitting a system diagnostic and either note/post it’s reference back here and then contact/chat with Sonos Support Staff via this LINK  and see what the Staff can suggest to resolve the issue.

Ok thanks Belly and Ken, much appreciated. 
I’ll give it a go