Ok, so you have a Session Stout. The ABV isn't the most important thing to be concerned with when you miss your FG. The bigger issue is that you could have an overly sweet beer (or an unbalanced beer relative to bitterness) because there are unfermented sugars in the wort (i.e., the yeast didn't finish the job).
There are a number of things that influence your original gravity and the final gravity. The first question you should ask is whether you hit your OG as this is a measure of your beer's potential ABV. Whether you hit this target will depend on a number of factors such as whether the grain crush is too coarse, the temperature that you mash at (mashing at the lower range generally gives you more fermentable sugars), how you sparge, etc.
FG is influenced by the health of your yeast, how much yeast you pitch, the type of yeast you use (some yeasts eat more complex sugars, others eat the low hanging fruit and call it good), the temperature at which you ferment, etc.
If you post your recipe, including the yeast, amount, and fermentation temps, I think the kind folks here should be able to help troubleshoot.