1. Verify that the Z-wave device is in range of the Devicebook Hub, or of a Z-wave range extender, using the steps here to estimate the signal strength of the hub at the device's location. If signal strength is the issue, add a Z-wave repeater device between the Devicebook hub and the Z-wave device.
2. If the device has usable signal strength, replace its battery.
Note that if a battery-powered Z-wave device is unable to communicate with the Devicebook hub, it may be draining its battery more quickly as it sends out signals trying to re-establish itself on the Z-wave network. This means that if such a device has been inactive or unresponsive for a while, it is very likely that its battery will need to be replaced.