This the second post I have seen with the same issue. 

Have you tried rebooting your phone or resetting the app?

I do not know if it is a known issue, so I would call support about it.

See my reply here:

Known issue, to be fixed in next app update.

Bit of a joke really.

This has happened before but the sub control was there and adjusting the slider had no effect whatsoever.

So frustrating.

Sonos pls don't update and release if they clearly don't work.

Anyone know of how soon the next update will be released?

Cheers.

If you look at the update notes page, they run between every two weeks, and approximately monthly. I’d assume that’s due to how much is in each release, and how much needs to be beta tested. 

Cheers mate!