Which reason could cause a contact to fail to enter a Journey Builder interaction?

The reason that a contact may fail to enter a Journey Builder interaction is that the interaction allows re-entry only after exiting, and the contact already exists within the journey. In Journey Builder, certain interactions are designed with specific entry criteria that manage how contacts can join or re-join a journey. If an interaction is set up to allow re-entry only after a contact has exited, and if the contact is still in the journey, then they will not be able to enter that interaction again until they have completed the exit criteria.

This design ensures that the pathways through the journey are clear and that contacts are not simultaneously processing multiple interactions that could lead to confusion or conflicting messaging. Therefore, maintaining the rules of entry and exit helps in effectively managing customer journeys and promoting a cohesive experience for the contact.