All information provided below comes directly from the manufacturer
Pairing the sensors
Nothing has changed in terms of software or hardware so there should not be any issues with pairing the sensors. Occasionally, we do find that some of the batteries inside the sensors stop working and should this be the case we will replace these free of charge - assuming its within the 2-year guarantee. If it is a brand new set, then it would be extremely rare to find more than 1 sensor not working. If the customer is unable to pair sensors at all, or has more than 1/2 sensors issues then there may be another issue. The sensors will either work, or not work, so there is no way to troubleshoot them as such.
As you may already know, the sensors switch on when they are upright and hit by daylight. Therefore, if someone is trying the pair the sensors in a dark room or led down, then they may experience problems. The sensors transmit a high frequency noise, so if there are issues with the phone's microphone or maybe they have a phone case blocking the microphone then the sensors may not be able to pair. Also, if they try to pair in a loud environment or next to someone else trying to pair their sensors, then they may have issues.There can also be issues if the customer does not allow all permissions as requested on the phone (microphone, camera, location etc).
You can run diagnostics in the app to check the sensors are working - always worth trying on a different phone to the one trying to pair. There are details on how to do this in ArccosAcademy.com as well a lots more information that may help. In short, head to 'Clubs', then 'Diagnostics'.