My suspicion is your head shape might be giving you more trouble than sizing. e.g. if you have a round head shape and try to fit in a narrow head shape helmet, even if it is actually the correct size, it might still create a pinch spot for you. Also, some brand, iirc like Arai, have different cheek pad thickness, so you have a little bit of adjustment there.
I guess the really true way to know is try different brands on.
Check these links out:
Helmet shape: http://www.webbikeworld.com/motorcycle-helmets/motorcycle-helmet-shapes.htm
Helmet fitment by Arai: http://www.shift518.com/showthread.php?t=29619&highlight=arai+leno
Hope this helps.
P.S. I am assuming you already and actually buy helmets base on the actual measurement of your head and per the manufacturers sizing chart.