When installing a smart plug for the first time there a few things to check:
1 - The smart plug must be plugged in and the switch turned on. The plug sensor has no battery, so it can only work when plugged into an on socket in the wall.
2 - If the sensor is in pairing mode but the Lilli hub is not, then you will have to put the Lilli hub in pairing mode. This can be done by pushing the centre of the top of the hub until you hear the click of a button. One of the lights on the top of the hub will then start flashing green.
3 - The first connection to the hub takes a little more time than the following connections, so to maximise connectivity, temporarily plug the smart plug in closer to the hub in order to help it connect.
If none of these help, please take a photo of the serial number of the sensor (found on the sticker between the prongs of the plug) and send this to support@intelligentlilli.com so we can remotely help with the problem.