In Mavericks why can't I change the password of an account even though I am admin and have unlocked the lock icon?

If the other account is logged in (possible if you have "fast user switching" enabled), you will not be able to make any changes to that account from System Preferences, even as administrator. To reset their password, they cannot be logged in. If you can't log that user out, you can restart the computer to force it to be logged out.