Best I can say is to contact Google. They may be able to help.
I just did that and unfortunately they said there is nothing they can do. I'm curious if when the tablet is wiped back to factory default settings and install, if that means the account associated with the device will no longer track it on Android Device Manager. While the account may not be on the device any longer, the device is still registered to the original account, which I would think would allow me to track the device.