Here are a couple things to try:
-Put all of your weight on the side with the release/tightening mechanism for the LAtch. If you push down on that side, you might create enough tension to allow it to slide.
-Recline the seat (as others mentioned) and wiggle the Nautilus side-to-side at the arm rests. You will have to use a little force, but it should loosen.
-Make sure you are pressing down on the gray part of the release/slider/tightening buckle thing. When ours was new, I had to use both hands the first couple times since it was tough to press down.
For future reference...the seat doesn't need to be 100% rock solid, never ever going to move. As long as you have less than 1 inch side to side or front to back movement at the belt path, it is tight enough. If you have to put your knee in the seat to get it tight, it's possible the seat is way too tight and could potentially cause damage.