have you spread your wings like a bird while using your jets at full power? It's physics based, so the angle of your wings is going to affect if you move up or down
I had some problems once when one of the reprojection options was on (interleaved?). The forces were not being applied correctly. Try fiddling with those options in the SteamVR settings.
If that doesn't help it seems like it must be you. Try replicating how I move in the gameplay video.
