Since we haven't heard about a general fare increase today, my guess is that either the lower fare class inventory is gone on your flights (due to revenue management or because others purchased tickets in the meantime) or you just missed an advanced purchase requirement (commonly 50 days for deeply discounted D1, 30 days, 28, 21, 14, 10, 7, 3, 1 and 0).

You can try changing dates slightly or looking at different routings and well as flights operated/marketed by partners such as VS, AF, and KLM.

You can also try calling. Some people report success in using an OLTA, but chances are that even if they offer the lower fare, it won't ticket.