I had the same problem on BKK- SIN a month ago. Steward tried to fix it and then said sorry. No voucher, no nothing. So complained on the website ( I am PPS). Got a $ 50 voucher. All the same though this is the first time in about 200 flts that I had this problem and their reply was very well drafted also referiing to the next flight I was taking and wishing me a safe flight!