I have used Spotify on my Logitech SqueezeBoxes for a long time now, but only recently got it working properly.
One problem I had was that I got a “not available in your region” error for many tracks. I had installed the Triode Spotify plugin rather than the official one from Logitech (which sucks by the way), but that didn’t help. I read somewhere that I had to select “always stream via helper” in plug-in settings. That prompted me to do a self-test (available on the settings page), which revealed a problem – the helper (on port 9005) was not available. It turned out the firewall on my server was blocking that port. I fixed that by creating a new inbound rule of type application and specified the full path of the helper (C:\ProgramData\Squeezebox\Cache\InstalledPlugins\Plugins\Spotify\Bin\MSWin32-x86-multi-thread\spotifyd.exe). After that, everything seems to work.