Speak with Master Gadrin in Sen'jin Village.
The venom from the broodguards wasn't as potent as the venom you brought originally. It must have come from a different spider.
I want a fresh sample of that venom. To find out where it came from, you must consult with a troll who knows the Witherbark well--Master Gadrin in Sen'jin Village. Travel to Durotar and speak with Gadrin. With luck, he'll know where the Witherbarks get their venom.