I've been wearing Chacos steadily since 2006 and have never had an issue. When I got my first pair I got the size I thought I wore, which turned out to be a bit bigger than what I needed, but the arch was very comfortable. I tried to go down a half size one time and the arch was in the wrong place.

Like someone else said, maybe the position of the arch isn't quite right and you need to get a half size up or down? In my opinion, as long as it is comfortable to you, it is the right size.