I had similar problems on the phone. My birthday is August but I needed to fly July 1st. On the phone I was declined twice, so I went to THAI ticket office and booked there, without any problems. I paid the taxes too so it would not backfire later on. Have to say that I'm completely satisfied with my journey this year on F from BKK-LHR-BKK and I can't wait to see if this will be available next year as well