re-installing sensor not possible

It was possible to install all 3 sensors with de smart indoor camera. But the camera reacted not as it should, so I reset the hole thing. The alarm and sensors were removed from de app. The re-installation of the alarm went as it should. But de sensors are difficult.

I have tried a dozen times, but no succes. De green light on the sensor is visable, but de camera cannot connect to the sensor even when I'm holding it next to the camera.

What can I do to connect these sensors?