My Network Monitor plasmoid suddenly stopped showing usage for the Ethernet link, but it still does for the Wifi. The System Monitor (ksysguardd) displays rates correctly. At some point, even though the plasmoid was broken I could see the graph in the NetworkManager systray widget. I tried another plasmoid (Netspeed?) and it had the same problem. /proc/net/dev has the information (I wrote a short script to use it).
Things could be related to a reboot that upgraded the kernel (currently on 4.13.0-37-generic #42~16.04.1-Ubuntu), but the Ethernet link kept its name.
Running Kubuntu 16.04
OK, not sure what happened. The plasma taskbar crashed and restarted, and the nes instance didn't show the monitor. So I added the widget again and the new plasmoid instance worked... but when I locked the bar's widgets again, the old plasmoid instance came back, still with no indications, while the new one was still there and worked correctly. So I removed the old instance, and so far, so good.