Why aren't any pending recipients showing up for the "first order shipped" trigger?
If you're not seeing any pending recipients showing up, the first thing you may want to try is increasing the sending window so that you can just look out into the future a bit to see that there are some pending recipients queued up. Then you can change it back to what it was.
If, even after using a quite large sending window (say, 100), you're not seeing any pending recipients queued up, it's likely because there's some kind of an issue with the order states. This trigger in particular picks up orders that have been moved to the complete state. If your orders aren't being moved to that state, they're not going to receive this email.
One way to troubleshoot that is by using this debugging URL:
It shows a breakdown of your order count in the last 60 days by state. As you can see with this example below, it looks like only one of their orders is actually complete and the majority of them are still in the new state.