Sometimes the firebrand skill just doesn’t applies stacks. It happens in absolutely random moments. You use your attack, you hit the enemy (the damage applies), but you get no stacks. Also it seems like the other effects might not even proc, idk. Relog fixes the issue. I’m playing offline. It seems similar to this issue:
I would add a log or something, but all of the links in a guide on “how to report a bug” are broken, so ask if you need more info.