This happened to me in both gauntlet and coop raid mode on three different heroes.
Savage: bronze skill was not charging at all
Gammond: bronze skill was charging erratically
Oro: bronze skill did not charge at all for one whole raid but worked fine in others
Seems to be a bronze skill only thing but happens across multiple game modes. Thankfully, this has not happened to me in pvp cuz that would suck.