I bought a used plastic Perception Shadow 16.5 sit-in sea kayak, which I am really enjoying, except for the fact that the seat back often won't stay where I adjust it. It has a sort of toothed strap on each side which leads forward through a device near my hip, which is supposed to grip the strap. Sometimes it does, sometimes not. Searching for seat backs, I haven't found anything that looks like mine. I'd welcome any suggestions as to how I can fix it, or what I could replace it with.