My child is calm but is taking a long time to fall asleep at bedtime - Silent Return

If your child is laying in their bed calmly and there is no interaction going on whatsoever (no parent checking in them, no talking through the monitor, etc.) then a late bedtime is likely the culprit.

Make sure you have any early bedtime based on the Perfect Bedtime Range guide. When kids go to bed too late, they get a second wind and their body fights sleep. They can be restless and unable to fall asleep even though they are laying down calmly.

With Silent Return, if your child still spends some time after lights out exiting the room, factor that time into the lights out time.
For example, if ideal bedtime is 7:15 but your child spends 30 minutes using their limits and then is finally calm around 7:45 but is restless and not sleeping until 8:15, they may be getting a second wind somewhere in between which is also contributing to their inability to fall asleep. So, try 7:00 for 5 nights and see if that makes a difference and helps ensure they are asleep before 8:00pm.