I have had that happen to me before also for previous trips. I just tried to look at the patterns of the previous years hours and the month prior. I think also the WDW dining people will know when Fantasmic is running because they book the packages for the fantasmic dinner package.
I have only been booking my ADR’s about 2-3 months out and haven’t had any trouble the last 2 years. But I also understand some restaurants are really difficult to book so the sooner you can book them the better. I also have more luck with dates and times using the online reservation system rather than calling in.
Good luck and happy planning!! I hope you get all the ressies you want and please share with us once you book them!!