That is a 3115/2. It should be submerged to the groove near the top on the side, about 10-11"
To get good results the water level has to remain constant and the flow past the skimmer must be gentle. In general some change is made to each Tunze product every 6 months, the products tend to evolve slowly and in ways most people don't notice. Do you think you can get a picture of the pump label and internals? This would give me a better idea, older skimmers were far more finicky and it may be a good idea to change some parts if it is an older one, or at least be sure the parts are working.