The receiver and controller may occasionally go out of sync. To synchronize them, plug the receiver into your device and insert a thin pin (e.g. an unfolded paper clip) into the small hole on the side: you'll feel a little "click" and, after a few seconds, the LED will start flashing rapidly.
Switch on the controller using the central button, but only release it when the LED strip is flashing rapidly.
After a few seconds, the LEDs on the controller and receiver stop flashing: they are now synchronized and you can use them as normal.