Overlord bug...

When you're fighting the overlord, it seems as though for every person, you can only damage him to 1/5 of his health. I noticed this once when it was 4 of us, and the overlords health bar stopped at 1/5 health left, and when I was soloing him after my whole party died, I was alive for a good 3 minutes yet I got him to 4/5 health, and I couldn't damage him any further. Also, sometimes when its 5 of us, and we pin him against the wall, we can't damage each other, we usually wait until a barrel spawns so we can make the barrel explode and he'll knock forward so we can hit him again. Anyone else notice this? Thanks!

Um maybe because he had a mana shield on?

*looks around* Huh? what? yes i was there...

Nope, you weren't there, and nope, he didn't have mana shield up, I checked, not to mention I couldn't damage him anymore for a good 3 to 5 minutes, that wasn't it, trust me, it happened 4 times to me total;)