I've found the last messages column helpful too. It's sortable. The N/A might also show a non-responsive device. My N/A's in the table below are zigbee plugs I unplugged as part of an experiment. I've also had a battery device appear once as N/A when it stopped reporting for a while. Also you can look at the date/time in a sort to see which haven't reported lately, and could check them out. I'm not sure what the ping every 3 hour option does. Maybe it assigns N/A if it doesn't hear back?
