Trouble Activating 6D

I am having a problem adding a 6D to an existing system. It is out at the perimeter of the system, so it COULD be out of range, but I do have a lamp controller a bit further from the Main Repeater than the 6D and that is addressable from a T10RL in the other room. Is there a trick to adding a 6D?

When I try to activate it using the serial number, it says "updating" on that device line then it says that 1 device could not be added by serial number. Adding it by holding the switch while in activating mode doesn't work...

Any ideas?