The problem could have one of a few causes...

1) Is the spring present or damaged?

2) Is the mechanism interfering with the pivoting of the coupling shaft?
This could be due to bent or deformed plastic on the chassis which impedes proper movement of the coupling shaft or due to contact between the coupling/shaft and elements of the chassis/body (buffers, etc)

3) Is the model equipped with lighting? If the lighting uses the plates for current conduction, it is possible that the plate is pressing on the coupling shaft mechanism

To check all three possibilities, I would recommend that you open the coach and observe the movement of the coupling shaft from the top of the chassis.
Once you have opened the coach, it will be easier to pop out the bogies (if required) and make any necessary corrections.

You should also verify that no added parts as shown on Page 5 of the manual are added to that particular coach as they can interfere with the range of movement of the coupling shaft.
https://static.maerklin....2b1727b7651464362121.pdf

As this set is over 10 years old, it will not be covered by warranty unless purchased new from dealer stock.

Regards

Mike C