Contacts are added into your campaign providing the campaign status is set to active and the contact meets the conditions for starting the campaign.
These conditions are set on the Start Trigger in the Campaign builder by clicking the Setup button:
Conditions can be combined with AND / OR rules.
If you have used condition reversibility on a tag within the start trigger, there is a chance that a contact can stop meeting those initial conditions while they're in a campaign.
The start campaign condition item has a triggers tab. You can choose what will happen if a contact stops meeting the conditions for the campaign's start trigger.
Choose Stay in the campaign and the contact won't leave the campaign unless they're specifically removed from it by a remove from campaign action.
Choose Leave the campaign and Pathfinder will continue to monitor the contact while they're in the campaign. If their Shopper DNA changes and they no longer meet the start conditions, they'll be immediately removed from the campaign, no matter what stage they're at. If the same contact meets the start conditions at a later date, they'll be added back into the campaign from the beginning.
Note: If a contact has been removed from a campaign by a remove from campaign action they will not enter the campaign again, even if they meet the conditions of the start trigger. This is true irrespective of which trigger reversibility option you choose. In this case, once a contact has been specifically removed from a campaign they can only be added to it again if they are added to it by an add to campaign action from within a separate campaign.