I see the issue. This is a result of a slight change in how triggers run versus the previous scheduled system. To avoid sending thousands of old emails, the system records a Last Attempt date on each trigger (you can see that value if you open the trigger under settings). If trigger events are created in such a way where the event date occurs before the Last Attempt for that trigger, the event won't run.
To correct it for your situation, change the Days Offset value to 1 day after booking and it will go out the next morning. This is, in practice, exactly the same as what you had. Before, doing "0 days" would basically go out the next morning.
Triggers are going to be expanded to include a lot of other event types as well as non-date events. So you'll be able to get instant sends for things like new booking or booking paid in full.
