I have done some tuning of our 3656 stockyard with some limited success, but nothing to write home about yet. They are tricky and each has a personality of its own. It sounds like your stockyard bridge is not quite aligned with the stock car or the little tabs on the bottom of the bridge are not making contact with the platform (where the cows stand) to translate the vibration from the platform to the bridge.
The trick is to do some slight bending and twisting of the bridge to get the bridge as plumb as possible with the car runway and also maintain contact between the bridge tabs and the platform when power is applied.
