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.
Ah, so you want fresh venom from the spider of the witherbarks, yes? Well, sit down... you're not going to like what I have to tell you.