There are a few steps you can take if your Sensor isn't connecting to the Bridge.
The first thing you want to do is to make sure that the Bridge is online. The status light on the back of the Bridge should be solid blue while it's online. (Will turn flashing blue when adding sensors)
If the Bridge is online, try connecting the Sensor while you’re near the Bridge. If the Sensor connects you can move it back to where you would like to mount the Sensor.
Next, try removing the sensor’s battery, then placing it back in. This is effectively Power Cycling the sensor itself and should fix any issues not corrected by the earlier troubleshooting.
Failing all of the above, try pairing the Sense Bridge with a different Wyze Cam. If the Wyze Cam which you originally paired the Bridge with has any communication issues, this can contribute to the sensors failing to properly pair.