We are in the same boat! Trying to get either August 6th or 7th. What does everyone recommend for trying to get them - check every day, a couple times? My daughter will be bummed but we can always open the park or close it to reduce our time in standby.
I don't think they were ever up for grabs. My trip starts Aug-4th. At 7:00am 60 days out I was online and booking. I was able to get BTM for AUG 4th but right from the start the 6th-8th all times showed "unavailable"
Also note: Big Thunder is scheduled to closed Aug 8 - Nov 12th.
I just got FP+ for Big Thunder for 8/7 this morning!!! I don't know when FP+ was added to the system for Big Thunder on this date, the date before the scheduled closure.. The availability of FP+ for 8/7 seems to be good...