When you want to use Sonos speakers as surrounds, they must first be unpaired from the stereo set they’re forming. See https://support.sonos.com/s/article/2927?language=en_US Could this be your problem?
When you’ve succeeded in doing that make sure you’re TV is playing surround.
By the way, even when they’v e been bonded as surrounds you will not get surround playing music from Sonos sources (i.e. Spotify). You will get stereo from the soundbar and stereo from the surrounds when the surrounds are set to “Full”.
The purpose of surround speakers is not to play the same sounds as the front speakers. It’s to play any “surround” effects the film etc might have to create a spatial effect, such as aircraft flying overhead. Only then will you get sound from the surrounds. And, of course, that also assumes the sound track is being sent to the Arc in an appropriate format.