I have never had a problem with Carbonize not working, until I got the pvp armour set. I picked up the Powertech armour set that isn't the Supercommando one, and have enough pieces to receive the +.5s duration bonus. However, after testing multiple times on other players, I noticed that the skill just doesn't work at all. I don't know if its just the set or me, but one time during a hutball game tied 2-2 with 10s left our ball carrier was going down, and I was out of combat so i actually dropped one piece of my pvp gear to run up and carbonize and voila, it worked.
As far as I can tell, the skill is broken with 2 pieces of the pvp set.