There’s a long thread here on issues with KP115s (and KP110s) going offline at times - not really a Sense issue, but more likely some kind of weird networking interaction between home network and the TP-Link Wifi stack.
If you look down that thread you’ll find a key to what NA typically means (a connectivity issue)
What I have found is that sometimes these connectivity issues can manifest themselves in the Kasa app as well (when you go to look for power, the Kasa app locks up) and sometimes not.