I'm wondering where the failure is: in the jaw/tongue area or the hardware that the coupler lock uses?
I'm thinking that if the trailer was rocking back & forth on the ball long enough, either the jaw or the hardware's been taking a beating. Could even be that someone using the couple on a smaller ball helped beat it up.
Stumpy might be on to something: measure the ball & see if that's a contributing factor.
I'm guessing the trailer has a lot of miles on it, if it wasn't abused by a previous owner/borrower. My 35-year-old open trailer with a 2" coupler is still fine, but it's lived a pretty soft life: always lubed ball/coupler, has only seen pavement & some gravel road, probably never loaded more than 4400 lbs on it, lives indoors when not being used.