We discovered that a few of these are misbehaving.
It should work if you do a reset first (press and hold the pairing button for 10 seconds, green light should flash) and then add it to the Almond+. Once you've added it, if you go to Connected Sensors, locate the plug, tap on edit and then remove, it seems to be reset back to how it should function and you should be able to add it without having to do a reset first.
If this still doesn't work, please email [suspicious]
support@securifi.com[/suspicious] and we'll arrange for a replacement.