I've been here before on more than one occasion.
The critical point for these DSCs is that you make a new booking during the "booking period" for a flight within the appropriate travel period. The QF IT system uses the PNR creation date as
@Matt_01 noted above. So when you make a new booking within the correct booking period but pay with a Gift Voucher, the QF system sees the booking as having been made when the PNR (for the Gift Voucher) was created and if this date is outside the booking period it doesn't apply the DSCs. This has happened to me on at least 6 occasions.
On each occasion I have rung QF and pointed out that the booking was made between the correct dates and the flight dates met the T&Cs. I also point out that the issue is that I paid with a Gift Voucher and that there is nothing in the T&Cs that prevented me using that payment method. The CSO has always awarded the DSCs manually without question. In your case, I would call again.