I had that problem with mine until I figured it out. You have two handles one under each side of the seat. You can pull one or the other to release it (you don't need to pull both). The trick is that you have to push down in the center of the seat and pull out on the latch handle at the same time. Kinda pull and push (jiggle) the seat around until it releases. Then just push it up until it snaps in place (full up position). I have a lot of stuff stored under my rear seat. As long as you push the stuff in there and wedge it tight it will stay and not slide out.
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