Make sure the release/holding clamp mechanism is opening up enough to allow the trailer cap to drop down on the ball without getting in the way.
One thing I do is to crank the 3rd wheel down until the cap's front edge is resting on the ball (sufficiently enough so that it won't slide off when I continue to crank the 3rd wheel up several more inches). Once I have the 3rd wheel far enough off the ground, I simply pull the boat forward until the cap drops down over the ball. That way the holding clamp is pushed "back" by the curvature of the ball, and not pressure pinned against the ball by the tongue weight of the trailer. Then I lift the holding clamp trigger and set it down into position to put the lock back in place.
OR -
When you lift the trigger release in the cap end of the trailer ... put your finger in there and push the retainer clamp back against the cap, and then see if it isn't easier to drop the trailer cap onto the ball. When they get rusty or grease residue traps dirt inside the cap ... that trigger/clamp mechanism tends to want to hang up and not open fully, not allowing the ball to easily slip into the cap. Been there