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?