Sometimes the card just stopped working. One time I finished my meal with my family and my card was o.k. We went to do some shopping and it did not work. I went to the bank which issued the card and asked a girl to check it and she and the credit card company had no idea why it stopped working. I got a new card few days later.
Go back to Air Canada, I do not like its service too. Other airlines are more friendly and helpful.