Hyon,
your problem is properly that you just send messages but you never track when the transmission is complete. which means that the messages for the end device is stored in the coordinator for a period of time as the end device is turned off.
the network layer will keep the message for 7.68 seconds before the message is discarded.
you need to track the apsde-confirm in your application and NOT send the next message before you have received the confirm of the first.
Br,
Mads