If you have Workspaces and Partitions set up on your instance, it is important to remember that people created in the database as a result of filling out Facebook Lead Ads form, will be automatically created in your Default partition per design.
In a scenario where a trigger campaign is hosted under a workspace that does not have the appropriate visibility to your default partition - the person filling out the Facebook Lead Ad form would not be triggering the campaign.
Make sure your trigger campaign lives under workspace which has visibility over the default person partition. To check the workspace configuration go to Admin > Workspaces & Partitions and make sure your default partition is assigned to the workspace where your trigger campaign lives under.