What is your source? From what app/device?

Also make sure the Digital Audio Out setting on the TV is set to Auto 1.

The source(s) are sky Q and firestick.  On checking more carefully at each program I can see that the output type , per the Sonos app, changes often depending on the program being watched, I guess to do with how they were recorded?

I will press on with changing my tv setting to auto 1 as suggested and see how that works.

thank you for kind help.

regards

tom

Not everything is mixed or available in 5.1 audio. Many sources are only available in stereo. Look for movies and shows on the popular streaming apps like Netflix, Disney+, and Prime Video. Those apps have a lot of 5.1 content. 

Also, be sure to set the Surround Sound setting on the Fire TV Stick to Best Available. On the Sky Q under the Audio Visual settings, set Digital audio out HDMI to Dolby Digital Plus or Dolby Digital.