I find there's a bit of a barrier in terms of when you start to bounce and how fast you can spin. For example, you might start bouncing in the saddle at 24 MPH and really be bouncing at 28. If you focus on smoothness and keep spinning, you'll break through the barrier and be able to reach a much higher cadence. I've also noticed that it takes me a few downhills for my legs to loosen up so I can spin faster.

Also, keep applying pressure to the pedals, don't let them pull you around.