If you like the Curt, I would install it again and do some testing.
A high hitch should leave more gap than the 1/8"-1/4" thickness of a plastic disk. Or the jaws wouldn't close. Can't really say from here, but lucky for you the salesman noticed something was amiss. Sounds like he didn't want to take any time to figure out the real reason. Now you know what to keep an eye out for, so you won't make that very common mistake.
That Reese is a good hitch too. A little paint on it, and you might find it to your liking.
Just don't listen to the noisy folks around here that tell you how their certain brand is so much better.