On a 9012 that will usually be because it is submerged too deep and the air is closed off too much. Between 1/4 and 1/2 of the front intake should be submerged and the air should be closed as little as possible. As you restrict the air you increase the water flow and the faster moving water carries out more bubbles. There are other possibilities like organic/surfactant additives and strong flow near the skimmer but this is where I would start. Unplug it, open the air all the way, reposition it and plug it back in. Then work with depth first and do not overly restrict the air, it should not be more than a few turns closed.
