The venom from the broodguards wasn't as potent as the venom you brought originally.  It must have come from a different spider.$B$BI 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.