By default, the system has the Contact Remarketing Pressure Interval set to just under 24 hours. This means that if we try to send an email to someone less that 24 hours after their previous send, the system prevents it. This includes subsequent stages of a multi-stage trigger.
Learn more in Contact Remarketing Pressure Interval.
So, if you have stage one set to go after 30 minutes and stage two set to go after 20 hours (for example), stage two would never be sent.
To prevent this happening:
Decrease the Marketing Pressure Interval. To access, expand the User menu and go to Settings > Triggers and Messaging integrations > Trigger Settings.
or
Select the Ignore marketing pressure for sends check box on the Trigger.
This may mean multiple emails are sent to someone within a 24 hour window.
or
Increase the wait time for the each stage to at least 24 hours after the previous stage.