WarmHam

I'm trying to complete the quest The Way of the Warlock: Part 3, which is where you practice a joint attack with Hajoon by using Tether Blade. I keep performing the attack when the prompt appears on screen, and it successfully knocks down the guard captain, but the system doesn't register that I'm completing the attack. The quest requires me to complete the Tether Blade attack three times against the Dusk Adder after Hajoon's Heatwave attack, and I've probably done it 100+ times. Is this a glitch/bug?