I know obvious question.. but is it running the latest firmware?
Spotify arrived with f/w version 4.0 & quickly followed 4.1 in 2014, we are now on 4.6
Are you using Spotify 320kbs ? If not that could be the issue.
Also for Spotify Connect (as used by 3rd party devices such as Naim) you need a paid Spotify Premium account rather than a Spotify free account.