How to use right the device variable CommFailure / PollNoReply / PollTxFail

Developer's Corner Scene Scripting
1

I am trying to understand the working of the device variable CommFailure / PollNoReply / PollTxFail .
When adding a Zwave switch device ( in my case the Everspring AN157 and AN158) these variables do not show up in its Device Advance tab immediately, but after a heal they show up.
I wrote some Luup code in a scene to send a email notification at a timer event if a device is not operating due to a power failure on its power line.
The code looks like this:

local switch=13
local lul_comm = luup.variable_get("urn:micasaverde-com:serviceId:HaDevice1","CommFailure",switch)
if (lul_comm=="1")
then
“send Email”
end

When my Vera V2 unit is set to change the on/off state of the device, but the device is not responding, then the variable CommFailure is set to 1.
When Vera is polling the device and getting no response the variable CommFailure stays 0, although in the UI4 the device is indicating having a communication failure too. Why ?
When the device gets its power again then I would expect to see the variable CommFailure back to 0 after a device state change by Vera. Sometimes this is the case but sometimes Commfailure stays to 1 . What it happening here ?.
I would expect also that the variable PollNoReply and PollTxFail would indicate a figure other than nil when the device cannot be polled. These values stays to nil.
The Alerter for Drowsy Devices tool developed by Ape15 makes also use of PollNoreply and PollTxFail variable to compute a failure rate. But in my case only PollOk is giving figures. PollNoreply and PollTxfail stay nil.
Somebody has a clue for me?