Eufy Flooflight Cam E340: not working at night while shielded from a glass

I put the cam inside my home and the cam monitors my front garden through a window glass. That is, the window glass separates the cam and the garden.

The motion detection detects me walking into a defined activity zone and sends out an alert during the daytime when it is bright. The cam is about 2-4 metres away from me when testing.

However, during nighttime when it is dark, the motion detection seems not to work with the same setup and setting including the same activity zone. If the floodlight is turned on, the motion section can send alerts, but it is after 6-7 sec, and not every time.

any idea?

Your issue is thermal variance. The cooler temp outside at night causes the glass to cool, the cooled glass masks the heat signatures that the motion detector relies on to sense motion.
Motion detectors use passive infrared. When a human walks in front of it, it detects that radiated energy and triggers.But when you have a cold surface in front of it , it can’t see past the cold.