If your current song file in Studio One is not mixing down properly, you want to make sure that every channel has its output set to the Main Output bus in Studio One. In StudioLive 16.4.2 and 24.4.2 templates, this is labeled as the "2 Track Return."
Not a Proper Mixdown and Project:
As you can see in this example, individual tracks are going to their respective StudioLive channels, not to the Main Output bus.
Proper Mixdown and Project:
In this example, all channels are set to output to the Main Output bus. This way the actual mix in Studio One will make it to the mixdown process and/or will go to Studio One Professional's Project page: