This exact same issue is happening to me. I cannot get the sync "beep" after holding down the sync button for 2 seconds. Instead I have to hold it down for ten seconds and I get "2 beeps". I believe this "2 beep" signal is to indicate that the device has been returned to factory settings. Thats what a support member told me on live chat.
I was also able to link it once when it randomly gave me a "1 beep" but like spunky_em it decided it no longer had battery - despite charging for the best part of 2 days on the homebase. I tried different chargers and cords all to no avail.
Maybe they should let you add the device using the QR code - similar to how I had to add the Homebase.
Really frustrating as I feel its just a software glitch but i can't restart the camera, and changing back to factory settings doesn't do anything. I am now told that I have to take it back to the retailer I bought it from only 2 months ago... This is going to be a drama I'm sure as I only want oine camera fixed out of a 4 pack. I can just see the confusion on the retailers face when they try to work out how to replace just one camera from a 4 camera box - or ewven getting the camera fixed in Australia!