End device not talking to former parent that was turned off.
You have an end device that is talking to a parent router (Node A). Node A goes offline or becomes unreachable by the end device causing the end device to find a new parent router (Node B). Now you have a network where your end device is a child to Node B, but no longer Node A, then Node A rejoins or reconnects with the network. At this point your end device sends a message to Node A, but does not receive a response ACK.
Why is Node A not sending a response ACK? Simply put, Node A does not have the appropriate route to send the response to the end device. When Node A was the parent node to the end device, it had the end device in its child table. Since the child table is non-volatile information, when the Node A rejoins the network, it believes that the end device is still its child. Because of this, it holds the message in the MAC indirect queue until next poll from child if the destination is a sleepy [RxOffWhenIdle] end device, or tries to send it directly to the end device. This polling does not occur and it cannot send it directly, so the message will never be sent.
Possible solutions include using either Route Discovery our Source Routing. The Route Discovery method will require for you to wait for the end device to timeout of the child table. This takes ~5 minutes (320 seconds) after Node A rejoins the network. Then if you have set the ENABLE_ROUTE_DISCOVERY bit on the global APS frame, then Node A will initiate an AODV route discovery and wait for the results. This method is less scalable then using source routing, since larger networks will have constant route discoveries occurring that will cause network traffic. The more scalable and recommended method would be to enable source routing. By using the plugin tab and turning on the 'Concentrator Support' you will be allowed to set up source routing with different options. It works by calling emberAppendSourceRouteHandler() [in EZN API] or examine current source route [in EZSP] to try to obtain source route from internal source route table.