It's Lily's strong will that kept them from working. Think of a love potion as a liquid form of the Imperius Curse with an order incorporated into it already. If the person who consumes the potion has a strong enough will, they can resist the potion. Thus, the Queen of Stubbornness and Strong Will would be able to resist even the strongest of love potions.

Thanks for reading and reviewing! I am glad you enjoyed it!