I have edited my profile at Stack Overflow many times. The edit form always reports as successful, but my name at Stack Overflow is still user905418 despite all my tries.
At the bottom of the page on your Super User profile:
Click the first one. This would allow you to use on Stack Overflow the username used on Super User, if both the accounts are associated to each other.
Have you maybe tried to change your display name too often?
The rule is that:
after a two-day grace period for new users, one display name change allowed every 30 days.