Your problem comes from the way the model is designed, the part of the model with the coupler is a free floating part. The simulated truck frames are not attached to the trucks, the only connection is the hinge point at the rear of the part so any strain (such as pulling a train) will force the coupler up. Fortunately there is an easy fix. The top of the truck and the bottom of the loco are smooth so i attached a small piece of double sided foam to the top of the truck. This was just thick enough to keep the coupler where it belongs. Be sure to remove the adhesive protection from one side only otherwise you will lock the part in place. And it is soft enough to allow some vertical movement when required.
I did have some concern that lateral movement might remove the top cover of the tape and make it stick to the chassis as well but that has not happened yet. If it does, just scrape it off and put another one on.
