A few questions that might help zero in on your problem:
- What yeast were you using and what temp did you pitch it at?
- What was your expected OG? Did you hit it? Do you know what kind of efficiency you got?
- Did you have any issues with the mash?
By my math, it looks like you hit 5.3% ABV with 66% attenuation. Depending on the yeast you used, you may only get another 1-2% out of it.
One thought: if you doughed-in at 160F and mashed at 155F, then you may not have had much beta-amylase and limit dextrinase reaction in the mash. This would lead to a lower-fermentability wort with more long-chain and branch-chain sugars (oligosaccharides and limit dextrins). A multi-step mash will help break down those sugars and make them fermentable.
If this was the problem, then you would see the long-chain saccharides react in an iodine test by turning a reddish-brown (as opposed to yellow with no reaction or blue-black with starches still present). Dunno if you can still do the test after starting fermentation though as I dont know how the yeast would react with the iodine.
Theory: Enzyme Breakdown of Starch
and The Sugars that you arent fermenting if you dont mash under ~145F at all
Practical Application: Multi-step mashing by John Palmer