Numerous people have brought up complaints about the autobrake not working sometimes for appearently no reason, myself included.
After tideous testing I found out:.
When the reversers are activated before the nosewheel is on the ground, the autobrake fails to work.
If you wait with reverser deployment until the nose gear is down on the ground, the autobrake works fine.
Oddly enough, I can hear the parking brake clicker when the autobrake activates. That´s how I immediatly hear if it works or not, even without looking at datarefs.
It´s all weird. I don´t see this behaviour on any other airccraft.
Tested about 100 times.