In Studio One, make sure that the mixer is open. On the main fader, at the top, there is a black box for routing. Expanding that will show you the places that you can send the audio for playback on the mixer. There is also a similar box on all of the channel fader. Make sure that all of the individual channels are routed to the main. Then make sure that the main is routed back to the mixer using the digital return. Lastly, make sure that the digital return fader is up in the main in UCS, and the main fader in UCS is up.
If you have the main output in Studio One set to any of the mixer channels, which is ok too, then those mixer channels need to be set to Firewire as the source.