Grenade does no damage?

[gossipboyxoxo]

I'm level 20 and I've just acquired the grenades. I've been trying to level them up for a few days now and have managed to gain all of 6 xp. So I'm still level one.
Personally I find them very difficult to use and I've noticed something. Following the idea of going to kingshead and taking out the "marching" officers (The ones in the group by the gates) I have waisted all 75 grenades a couple times now without doing any damage. I'd position myself behind them, throw the grenade, which would land right in the middle of them and then the little -250 damage markers would come up as if I were doing damage. After completely using up my 75 supply of grenades they'd all still be alive and lo and behold... all still have 100% health. I've tried this on several servers all of which had the same problem. Has anyone else had this problem? And how did you guys rectify it?