The payrate for this one changes dramatically. One month it can be very high, another - very low. If you find, that in the given month it's low, don't waste time on it and wait until the next month.
The coinbar has a small bubble on it that resembles a fuel gauge. While surfing it slowly runs down towards empty. As long as you have some "fuel" in the gauge, you are earning. If it is flashing slowly, you are no longer considered active by the coinbar, so you are not earning. Just move your mouse cursor over the banner ad to refuel.