From what I can tell, with the change to Firebirds, they also changed Archon so it deals the damage type of the Archon rune you use. So, good luck doing any damage with anything but the fire rune, and good luck with mobility if you do use it.
YOu need three fire skills in archon form to maintain 6pc bonus, that's why. You need
Combustion rune, as well as magic weapon - ignite.
Had the same problem with it not working, I feel dumb now.
Archon always dealt the damage type of your highest elemental damage when we used the set-bonus to give it the effect of all runes. Since the firebirds archon build doesn't use any Vyr's pieces we have to actually run the fire rune.