Nanlite Fresnel Lens for Forza 300 and 500

Unless I simply got a lemon, this fresnel does not work at all with the Godox SL300II. Even focused as tightly as possible, it does little more than a standard silver reflector to shape and concentrate the light. Now, before claiming this should only be used on a Nanlite, please check the Godox SL200II review on Gaffer and Gear, where it is shown to work beautifully. Ok, maybe there is something very different about the SL300II, but it is supposed to be nothing more than a power upgrade over the 200 watt version. Included barndoors are effectively useless, as they can only dim the light, not shape or direct it at all. Regarding the barndoors, while the design is clever, the leaves are very difficult to place precisely, as they tend to stick just out of position, then droop back if pressed a little harder. Perhaps I did get a bad lens on this. It does have a little bubble, about the size of a grain of rice. Maybe it's just bad glass?