Left side of Citroen C2 seat not locking?

My passenger Citroen C2 seat, does not lock on the left side, when I raise it. Therefore the recline mechanism doesn't work too and it stays locked in only one angle/position. I can fold the seat forwards and slide it back and forth, but when I return it in its normal position, it only locks on the side of the right handle.
Do you have any tips on how to fix this problem? Or at least a way to open the recline/locking mechnism of the seat? I can't seem to manage to do that too.