Connection lost for only a few devices, when connecting multiple devices in a rosserial_xbee network

I have a xbee network of about 8-9 robots with xbee devices and I notice that whenever i use`rosrun rosserial_xbee xbee_network.py /dev/ttyUSB* 1 2 3 4 5 6 7 8 9` and so on, to establish the network, last 1 or 2 robots always fail to join with the error `[ERROR] [1538767384.068548]: Lost sync with device, restarting...`. When the robots are restarted or the xbee network is reset, then all the connections eventually are successful, but it still takes a couple of trials [(on/off) of robots and xbee coordinator]. Why should this be happening? How to ensure that they dont drop out of network while I am communicating with them? Now, I have read up that the rosserial_xbee errors are usually extremely generic, but how do I ensure a reliable network with about 15-20 xbee end devices?