I have had on and off ability to login to my SkyMiles account for several weeks, finally resulting in seemingly being permanently locked out. Called Delta and was informed that my account 'was under audit'. Asked to hold for several minutes, and then told access was now available to me. No apparent reason given for 'under audit'. Only reason I can think of is I booked an upcoming trip in first class, which I have never done before - but that didn't make sense as I paid cash for it!