Originally Posted by Soju
According to the Thai website booking, BKK to CAN e-tickets are available but not CAN to BKK. Just an educated guess: the Chinese government prohibits the issuance of e-tickets for outbound flights. Perhaps because Chinese travel agents report all outbound passengers to some agency prior to travel so they can check up on them. Either that or Thai Airways isn't setup in China for processing e-tickets.
It could also be possible that the ground handling company that TG uses in CAN does not have access to the TG e-ticket database.