Z-Wave network is a mesh based network, it means that every DC/mains operated can widen it’s range by repeating signal.
To improve range try to place some DC/mains operated devicess (e.g. Wall Plugs) between battery operated devices and Z-Wave gateway.
You can also try to move Z-Wave gateway in a more central location.
Remember: Every wireless network’s range is highly dependant on construction materials used.
Caution: Do not modify antennas of devices in any way!