This bugged me for ages. Solution, make sure any other controllers are turned off. For whatever reason when both my DFGT and DualShock are on at the same time, the zoom function only works on the DualShock. Doesn't matter if I set the DFGT to be controller 1 or controller 2. But if I turn off the DualShock, then the right paddle works for zooming!
Vice versa also applies for steering. When both controllers are on I can accelerate/brake, change gear, etc using the DualShock but I can't steer. Switch the DFGT off and I can steer again.