On this page of our game guide to Batman: Arkham Knight we explain how to investigate the reason for losing contact with Poison Ivy. Batman has to reach a large plant where he will meet with Ivy.
Listen to another conversation with Alfred, then bounce from the roof (temporary you can't use the Batmobile) and glide towards the main plant of Poison Ivy. Once you reach your destination, interact with the plant and you will meet with Poison Ivy. After watching the cutscene in which the poisonous cloud will be neutralized, wait until Alfred makes contact with Batman. He will give you an opportunity to participate in some optional quests, but if you want, you can ignore them and continue the progression in the main storyline.