Has happened the last 3 times I flew out of LAX - I was going to JFK each time, but with Diamond status it's suppose to be automatic at the 6 hour mark.
I was reprinting my boarding pass, so not sure if that took me off
I just have the ticket agent add me back on the list - and cleared
Has also happened on occassion at LGA and BWI
I think the first time was about 4 or 5 months ago