There are several reasons why surrounds are low or no volume:

• Audio source is 2.0 or 2.1 which has no surround channel also known as stereo
• Audio source is 3.0 or 3.1 which has no surround channel

The following may require you to open the Sonos app for the room and adjust the surround level. The second number represents the surround channel:

• Audio source 3.2.1 also known as 5.1
• Audio source is 5.2.1 also known as 7.1

Other adjustments to make:

• Use TruePlay inside the Sonos app to tune the room
• Set the surrounds to play at full stereo for music and revert back to ambient for movies

The last assumption is that the room is not properly setup. If it is setup properly you should see an option to remove the surrounds as shown below:

If none of what has been suggested is helpful then I suggest you run a diagnostic within ten (10) minutes of hearing no sound from the surrounds. Post the reference ID in your next post and call Sonos Tech Support to discuss the findings.