Sounds like your coupler did not release. You need to lift the little lever up on the trailer coupler before lowering on the ball. If it doesn't rest in place, the trailer lever did not release the "jaw" inside the coupler. 2 5/16 inch ball is the right size. I just check the Coachmen Web site:
PDF click here.and you have the correct ball. I think the jaws inside the coupler are sticking.
Sometimes (rare, but it does happen), the jaws will close shut before the trailer is on the ball. I have to rise the tongue and that usually is enough to free it up from being stuck.