It's done. When it's done, it's done. Many extracts have poor fermentability. That's all this is. If you use the same brand of extract again in the future, trade some of the extract for plain white table sugar and you'll get closer to the final gravity you wanted. Or, try a different brand of extract next time. Also, be aware that Windsor ale yeast is a poor attenuator around 60% anyway, so that's why that didn't help.