You might look at the door lock on the panel. It may be adjustable some to allow it to unlock and also lock. I had this issue on my W128. The other item you might look at is wear on the “cam” on the handle that throws the lock on and off. If the other side works well, compare the profiles. Again on my W128, the “cam” was badly worn when compared to the other side I had the problem with. If it is worn, you can probably get an expert welded to build up using a small TIG. That is what I did and now they both work well.
Good luck.