The only thing to do is CALL them. There are a number of reasons why this happens - the one that happens to me most often is that schedule changes include a flight number change and seat assignments and upgrade requests require a match between the e-ticket and the itinerary.

It can also happen when your travel agent doesn't complete ticketing properly - again causing a mismatch between the eticket and the itinerary.

If you don't call them you may well find that you can't do online check-in or even Easy CheckIn at the airport.