Did you only look at booking with free dining or did you try a room only booking? Disney only releases so many rooms with each promo, once those rooms are booked you may have to look at another resort or try booking room only. I just looked online and could book room only at POR for 12/10-12/20.
December is a good time to go and the earlier in the month the better. However, Disney is doing a great job of filling empty rooms with good discounts. I think the slower times are getting busier every year.