I have to agree with the different brand v different head shape theory.
I can wear AGV's and Arai's but can't get a Shoei to fit. The medium is too small and the large too big. If your buying a new one watch out for the date stamp inside, remembering your lid is good for 3 - 5 years. They will all discount helmets that are getting on a bit and you may get some good buys but watch that it's not too old.
Cath
I don't know that your chin situation is too much of a problem, so long as the chin part isn't coming into contact with you because the helmet is moving around. It's probably something to watch next time you buy a new one.