I only noticed this yesterday night when I was about to level my monk.
It happens that I have a +6 DEX ring, but in my character sheet, the bonus is as she was wearing a +4 item. I removed it and placed it again, but made no difference. I will try to use another +6 DEX item instead to see if it works.
Meanwhile, I thought I'd share this situation with you, maybe someone has a similar issue?