Have you tried WD-40? It may just need some lubrication.
Also, sometimes the fit it just snug enough that to have it drop on the ball or pop off the ball, you have to back up or go forward with the tow vehicle just a nudge for the connection (with the trailer tires blocked).
I had to do both those things with my old shasta trailer - the WD-40 once real good, and then the other most every time I hitched and unhitched. I swear the round hole was slightly off shape to the ball. It did make me a little more at ease that it wouldn't pop off the ball while driving.
