Even if the collar did slide back somehow, how did the spring-loaded coupler gate manage to re-close so the collar could slide back into place? I can't see it. Either the ball doesn't fit the coupler properly (coupler socket or ball damaged or wrong size), one of the springs is broken, the ball was never in the coupler to start with, or the coupler wasn't latched.
Now I'm wondering if Oliver taught us to use the Bulldog incorrectly when we took delivery. He had us lower the coupler onto the ball with the coupler latch handle in the *down* position, then use a foot or rubber mallet to close the coupler around the ball. At that point the collar snaps closed and we put the pin in. Now I see there's another (better?) way to do it.