Originally Posted by
TerryK
I concur. It reads to me that AA failed to push over eticket to UA. UA refused to transport passengers without tickets. Sounds logical to me.
All airlines system will print boarding pass without eticket in order to accommodate paper tickets (yes they still exist). UA boarding pass is only valid for boarding if there is a 13-digit eticket number printed on it. Without it, a paper ticket or FIM is required.
This. The ticket has to be endorsed over to the new operating carrier for a ticket to be generated.
I had this occur when a gate agent improperly reaccommodated me on AS. I had to chase back and forth
three or four times 
to get the proper endorsement to occur, as the AA GA kept on.tell I.g me it was done and I had to courier instructions on what actually needed doing from the AS GA. Made the flight, in the end! (Fortunately, they had gates across from each other in SMF Terminal B - but that option won't be easily available when AA moves to SMF Terminal A tomorrow night.)