This may be due to a common misunderstanding of how the Night Light works, if it the mode actually works, but just won’t follow a schedule…
The important note is that the switch does not actually turn the Night Light on or off. What it really does is it turns the automatic schedule on and off. In other words, if the switch is off, Night Light mode is disabled. It will not come on at all - the schedule will be ignored. If the switch is on, it should follow the schedule.
So, to have it come on and off automatically, the switch needs to be left on, and the times need to be set. If using the Sunrise to Sunset schedule, Location Services in the Privacy Settings needs to be on.
If the switch is being kept on, yet the night light still does not turn on and off at the desired times, then I apologize, I am not sure what that issue would be.