Thanks, guys. Many of you seem to be saying similar things about the palm staying connected to nage's wrist, and this is what we are taught as well.
to avoid losing contact between your palm and nage's wrist, your body has to move.
As a solution/diagnosis, needing to move my body as uke makes perfect sense. If it's uke's job to stay connected, and there is some separation, then when nage moved, uke must not have or didn't move fast enough in the correct direction to keep up.
I'll try to "move" tonight. This is only an issue at higher than regular speeds, but perhaps that can be arranged...