This doesn’t make sense to most people, but sometimes the sensors won’t associate if they are too close to the base station.
If you get an error message that says the signal strength is too high, move the sensor away from the base station, hit the Search Unassociated Tags button and, if the sensor is found, the Associate button.
If the sensor drops out of “association mode” while you are doing this, you will need to take the battery out and put it back in to reset the sensor.
Replacing the battery is quite a quick process, but it can be a bit fiddly.
Here is a video that explains the process…
Watch on YouTube