I built a simple NetMonitor system to monitor Internet and my LAN devices.  Done via the PING command using ESP8266Ping.H library which seems to use the core ESP8266 Ping_Start( ).
Overall the system works.  The The Internet segment is fine and alerts if Internet if offline.  However – I have about 35 devices in the home – both purchased and built which operate or monitor various items in the home.
Of those 5 of them are “challenged”.   To keep this short I will discuss two specific devices.
The first is a commercial Chamberlain Garage Door Opener (GDO) – I have TWO of them and both communicate via the same Access Point (AP) on my network.  Their IPs are MAC assigned by my Router.  They talk to a cloud service and are FINE (i.e. for the point of this discussion they are attached and functional).  
The other is a Freezer Temp Monitor that I designed and built (ESP8266 based).  I have two of them – they are on same AP and talk to a Blynk Cloud service (again they are fine relative to the cloud service).   And the freezers are running identical code.
I setup the NetMonitor to Ping each of my devices every 8 hours – just to confirm they are online.  The interesting thing is that ONE of the GDOs and ONE of the Freezer Monitors fail repeatedly to respond to the Ping.   But as commented they are connected to their respective cloud services and working.  And 8 out of 10 times – if PINGed via my PC they will respond just fine.  And the “partner device” never has an issue.  
The Ping Code does a single ping within a loop to attempting 8 times 1 second apart (i.e. first good one exits) – if that does not work – logic waits an hour and tries another 6 times before “claiming failure”.  There is a 30 sec delay between devices within a given cycle (i.e. the 8 hour cycle time).
I have changed IPs via the MAC assignment.  I have moved things physically a bit (mostly the router).  I have changed the PING order.  I have altered the delay between devices – between pings – and changed the number of attempts.  Nothing has shifted the results.
Now an interesting comment.  I have a SerialMonitor program in the NetMonitor – which allows me to manually do a PING of a desired IP.  IF I ping the device from my PC – and it works – and then Ping it from the NetMonitor – it works about 80% of the time.   Only difference – TMK – is the PC is hardwired – the NetMonitor is WiFi.
I was researching this issue and found a thread that indicated RNT had done some work with the PING and Net monitoring idea (I could not find a direct link though).   As I did learn – I am NOT alone with this odd challenge – and like me nothing seems to resolve it.  
Anyway – thought I would ping RNT (pun intended) – to see if you have played with or heard about this – and if you knew any solutions or recommendations.
Thanks – and have a Happy Hoiday