speaker volume issue

Are you only using the Sonos app for control or do you have other controls by other services?

Is this a new setup, or is it a change of behaviour in an established system? If the latter, it may be a fault in a speaker. 
Generate a diagnostics report and speak to Sonos support. 

Thanks for the reply. But I noticed the new 300 speakers were sitting on tables in what is apparently an unsupported orientation! So by simply rotating then to the correct orientation the volume is now working ok. I know this seems odd, but it worked. 

I never think about the Play 5, Five and Era-300s being aware of their orientation, interesting to note the 300's reaction.