Did you do a zigbee heal after you added the plug? If not, it may just have taken some time before all the network address tables were updated. See the following:
One more thought. While I understand the marketing reasons for the decision, the fact that SmartThings gives customers zero guidance on how to set up a network backbone does lead to a lot of frustration that shouldn’t have to exist. if people knew that
.1) The hub should be located centrally in the home, both vertically and horizontally. Don’t put it in a garage or basement as those have concrete and pipes and metal objects which will reduce signal. If your Internet router is off in some inaccessible corner, you can use a Wi-Fi access point that has a ethernet port on the side and plug into that. Also, the hub should not be put in a cabinet. You just want to make it as easy as possible for signal to spread around your house.
.2) you need one device that can repeat about every 40 feet…