Welcome to HE! 
For every 6, or so battery devices, you should install a mains power repeater. Here is the HE KB about building a solid Zigbee network. Be sure to add a repeater "inside the network", before trying to "extend the network" too far.
I just got a few of these yesterday to boost my Zigbee mesh. After adding 40 iris battery sensors with the deal @ogiewon posted here, I am also having an issue with some delays, device falling off. I have not used those repeaters yet, but there is also a post about good zigbee plug repeaters, and more here.
