Improving Z-Wave network reliability

My Z-Wave network has been pretty stable for some time now - in the sense of its members and physical layout. There are a bunch of power switches around the house and four radiator valves. Nothing sophisticated. I have migrated to OpenZwave, but other than that, and changing the occasional battery; nothing has changed.

However, of late the network reliability has degraded. I’m getting an increase in the number of cases where commands fail: e.g. to toggle a power switch or change a radiator setpoint. None of the nodes have failed, but I’m seeing an increase in the number of failed message sends on some nodes.

I forced a network heal a few days ago, but the situation hasn’t improved. Other than adding some more powered nodes to improve connectivity, is there anything else I can do?