Why won't System Preferences in OS X 10.10.5 accept my username?

This extremely vexing problem has recently started where I can log into my Mac Book just fine with the username and password I've always used, but now whenever I want to change something in System Preferences and am prompted for my username and password, it simply will not accept my username. I know the password that I'm entering is correct as the password hint it gives is exactly right. So I assume that the problem is with the username--though the username I always use works just fine for logging into the machine. By the way, this problem did not start until I turned on the parental restrictions, which now I cannot alter because the username I enter does not work.

If anyone could shed any light on this, I would be eternally grateful.