Get a rubber hammer underneath and give it a few taps on each side runner. Mine got disorganised and did the same. I saw another post soon after so its not totally unknown. I couldnt see why it was stuck and took the seat out. Still couldnt see why and put it back after clouting it and making it work again, its not done it again, The link between one side runner of the seat and the other is a bit tenuous and I think giving it a clout or two will likely do the job. Its not hard to get the seat out if it still doesnt repsond to a bit of the Manchester screw driver treatment. You can then sort it out and reinstall. It only took 30 minutes all in.