This behavior happend to me too. It’s hard to say how often, because I noticed it from time to time, but wasn’t sure about. Today I caught it:
On this picture you can see normal discharging until I plugged it in (USB-Hub of monitor, where its connected every day for charging). You see the green bar at the bottom (“wird geladen”) but the percentage dropped as it would be unplugged. When I noticed this, I tried to get a screenshot but shut down the phone by mistake:-). Then there is some time of using without charging, and the last minutes (after reconnecting to the same USB with the same cable), it started to charge.
So we learned:
The red LED was on.
The cable is OK.
The power is enough.
It discharges although, even a bit faster than before (longest ramp), although without WLAN, without GPS and without Signal.
After reboot, it charged as normal (which I already supposed, based on earlier experience).
There is a second thing to be seen which I don’t understand: This curve started after charging the whole night, at 100%. During the first hour, I used it rarely, but it discharges quite fast. Then, without a reason (as far as I know) it discharges slower. The next fast discharging is ok, I used it for navigation and forgot to plug in to the power supply in the car. But this is a quite heavy load (display, GPS, Bluetooth and data) and discharging is as fast as in the morning without doing that…
Roland