The trick on the Disney website is to know in advance which days Fantasmic is showing on the months that are not showing up (this is a glitch in the Fantasmic booking system...those months show up for everything else) Anyway, check at the wdwinfo.com page for fireworks schedule to see when Fantasmic will be going on 5 & 6 months out:
http://www.wdwinfo.com/wdwinfo/hours.htm
Once you know what date you want, go to the Disney site and choose Dining Events on Where To Eat, then scroll down to Fantasmic section and click on Make a Reservation. When that comes up, in stead of clicking on the calendar and trying to select a date,
type in the date, using this format: 00/00/0000...so if you wanted Sept. 4, 2011, you would type 09/04/2011. And then click lunch or dinner, your preference, and put in party size you want. The restaurants available will show up. I actually just tried 9/4, which is the Sunday of Labor Day weekend, and there are still several choices still available for a party of 4 at 2 of the 3 restaurants even though it is way less than 6 months out. So I think this glitch in their system (not showing 5 & 6th month out) is maybe keeping the bookings lower than normal.