Was this an extract beer? Depending on the manufacturer of the extract, it can have a lot of unfermentable sugars that will never let your beer get down to less than about 1.018. Next time you could try a different extract manufacturer or kit maker. For this batch, I'd consider throwing in a pound of sugar to jack up the alcohol and thin the beer out a bit. Regular old white table sugar is 100% fermentable so it will bring the alcohol up to where it needs to be for the style, although yes it could dilute the flavors somewhat since it is essentially flavorless. You could add a pound of honey or something like that instead if you wish. Not saying that you have to do this, but it is worth thinking about, because if the fermentation quits at a gravity of 1.020, then your alcohol is only at.... about 4%... which I suppose isn't too bad. Up to you.