Not all programs are transmitted in 5.1. The app will show you what signal the Beam is receiving/playing. Also, the Beam will handle Dolby Digital but not DD+ so check that on the tv settings. Finally, do you have the Ones bonded as Surrounds or did you add them to your system as a stereo pair? 
Let’s start with that and see where to look next. 

If you have the speakers bonded in a surround setup music played from Sonos sources is not surround but stereo (on the Beam and on the One SL’s)……..