It probably is charging, but has turned itself off in order to charge faster and therefore isn't updating its battery % in the app.
The button on the tracker will glow red while charging and blink red when charged.
After charging, unplug the USB cable and press the clear button for 2 seconds until the green and blue lights flash. In a few moments, the tracker will update its battery % in the app.
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.