This is a reoccurring problem with non-MM couplers. There are other threads on this issue on the forum here. Regardless, click
here and then on the "replacement parts" tab about 1/4 down the page. Find the check ball and check ball retainer part number and order form MM.
This link is for a D coupler so if you have a different coupler, go back to couples and click on what style you have and then follow the same steps. Most have the same parts when it comes to this (D&S couplers).
Cheers,
Steve