Uncouple the trailer and insert a spare ball into the coupler (a). Close the coupler latch lever. If the fit is too loose, remove the ball, and tighten the spring-loaded nut (b) on the underside of the coupler.
Reinsert the ball and check the fit. Repeat the latter part of Step 1 until you get a fit that is snug but not so tight that you cannot move the ball by hand with reasonable effort. Rehitch the trailer.