I contacted fitbit for help on the same thing, they told me to try and clean out the little hole on the side of the device and then long press the button to restart the device.
This provided a temporary fix even though it "registered" my heart rate at 68 no matter what I was doing (sitting OR exercising) so it didn't REALLY work. As of this morning the HR sensor has stopped working again.
It may work for you, though I kind of doubt it.
Best AnswerNope still not working
Best Answer