I have been using an old iPad for Zwift and my iPhone as the companion. Sadly I think the iPad is too slow as it kicks me out when there are a lot of people on the screen. When I try to use Zwift with my iPhone (10X) it connects to my sensors but the avatar does not move at all when I try to ride.
Help? 
So I spent another long while on this last night. Replaced the batteries, tried another iPhone and still no success. Finally went back into a ride with the iPhone and was still not loving my avatar but within the ride went to settings and removed and added the sensors again from inside the ride and it worked.
Have yet to try it again to see if it stuck but it appears that resetting / adding the sensors when a ride is already started woke it up somehow.