It is very common for W123 seats to have limited rearward travel when a coin has become lodged in track. Pennies, dimes and nickels are just some of the things I've removed from jammed and malfunctioning seat tracks. Get a good flashlight, dental mirror and flexible grabber retrieving tool. Inspect from front and rear ends of seat. You may be surprised what is causing the interference. If nothing else, a good visual assessment will confirm locking tabs are moving sufficiently and properly disengaging when lever is actuated.