Passing down by your knees make the pass shorter, hence easier. This really helps when taking a bs 360 to bs 540. My experience I would bet that when you are trying to take the trick bigger your are having trouble with the line tension. Fighting the line tension will pull you off axis the wrong way. I would suggest taking a longer, bigger approach. This will allow your to maintain less line tension, get the speed you need to take it bigger. A lot of BS spinning has to do with body position at the wake, you want to be standing tall to manage the line tension. A longer approach will allow you to get the speed to take in into the flats with having your compromise your body position.
Jeff Makes som e great points about weight distribution. For me spinning backside I am fairly even footed. Also, really focus on bringing the trick up and the wake opposed to out. the increased speed on the approach will want to send you farther into the flats, you need to use that energy to send you up as well.
Hope this makes sense.