removing back seat from renault scenic?

hi I have removed all three of the back seats form my scenic and moving them so there is just two in the back but closer. one has gone in fine but the other one has got stuck. only one side of the seat has locked into the base of the car. I can now not remove it or fix the other side of it down - meaning the seat it now unsafe to use.

any advice, I tried releasing the handle as normal but this just flaps, Ive moved the seat into different position but nothing