I have that with my passenger seat, from what i could see it somehow got knocked out of place so that the mechanism wouldnt quite fit into the right place, i.e. the hook thats meant to clip in is ever so slightly out of line.
Anyway tried banging it with a hammer, bending it with a wrench etc but none of these worked.
Three ways around this, very heavy person sits in it, small person jumps on it til it goes in, or thirdly, and better you put the seat as far forward as it goes and then clip it into place as it then seems to line up again when it is moved to that position.(not always necessarily as far forward as poss, but until it clicks into place)
HTH