Turn the car over, look at the coupler mechanism. If it has a spring running lengthwise, the spring tension is not enough to keep the coupler closed. You can most probably open the coupler by pulling on the end of the coupler. If so, replace that spring with a stronger spring, or elongate the installed spring, cut the excess off the spring and reinstall. You can get different springs at either Ramsey or Ace Hardware stores.
Bruce