I would suggest a slightly different strategy. Book the number of seats you can get at the lower price. Then wait one day to see if the computer releases more seats at that lower price. The computer looks each day (actually who knows what frequency) to compare the number of seats sold against what it's expecting to have sold by this point and adjusts the number of seats available at each fare bucket each day.

You might also try to buy some from other vendors like expedia or orbitz. The processes they use to book tickets do have a certain amount of delay and cached numbers in them so they might have more seats available or they might not clear their inventory when you buy the seat on ac.com.