Maybe see if the below might assist…

Sonos App User Guide:

https://www.sonos.com/en-us/guides/sonosapp

And perhaps see the attached from my own Sonos App…

Sometimes the volume slider will be missing from a speaker in the list, which makes changing its volume challenging.

I’ve not seen the issue @controlav mentions, but note the two different screens/forms shown in my earlier attachments. I’ll just refer to those screens as:

• The Slide-Up Room/Group Selector Screen
• The Output Selector/Room Grouping Screen

In the latter screen, the volume slider control only shows when a room is selected (checked). See the attached example taken from my iPad controller App. Hope that assists too.