I too have noticed the same thing with fury but only when I am svsing. I just figured it was someone using the glitch but once I hit them with fire then switch back to fury it works just fine. And someone did mention to me on here that to check "take cover" and most of the time that is what it was but a few were using the glitch so I would just lure them out away from those glitch rocks and then take them down without any problems. This is just what works for me but yes, check the next time and see if "take cover" was activated and if so hit them with fire then switch back and see if fury does damage.
Kani of Spartan Ellite