We have an .Evenflo system, and it did the same thing. We tried putting it in the center of the backseat, and it would move more than an inch in each direction. Tightening the seatbelt did nothing to stop this.
What fixed it was putting it on one side, instead of in the middle. They don't really recommend that, because of a side-impact danger, but there was nothing else we could do. The dumb thing wasn't secure!