By design, complications only display overdue reminders. It shows the last overdue reminder in the past 24 hours, thus acting as a passive “notification” for overdue reminders each time you look at your watch.
If there are no overdue reminders in the past 24 hours, Due will not show any reminders in the complication.
This is much like how the app itself works: you set a reminder for a certain date and time and forget about it until it becomes due. Then, Due will relentlessly let you know that something needs to be done through notifications and auto snooze.
If you'd like to see upcoming reminders on your Apple Watch Face, you can consider using the Siri Watch Face. Due integrates as a third-party data source to Siri, and if the Siri determines that it is relevant to you, it will show you upcoming reminders from Due.