It sounds a little like an out of synch team. When you ride your singles together are your cadences similar? Are your shift points similar? I've ridden with a lot of stokers and I've ridden stoker myself and the simple truth is that not all teams are automatically fast on a tandem. Teams that by luck have similar riding styles have an easier time of things. I've ridden with a few stokers and have sworn that the brakes were dragging and others that I hardly knew they were back there except for the fact that we were going very fast with not much effort on my part. Sometimes it just take miles and practice.