Your tracker may be charging, but has turned itself off in order to charge faster and therefore isn’t updating its battery % in the app.
For the Mobile 200, the button on the tracker will glow red while charging and blink red when fully charged. You will still need to press and hold the button for a few seconds until the blue and green lights flash to turn it on.
You can always confirm your tracker is on by pressing the button once quickly. If the tracker is on, the lights will flash for a few seconds.