I saw a thread on tastybrew where you also posted this same issue. I think your best option is to a) get another thermometer as backup and b) keep increasing your mash temperature until you get an attenuation you want.
80% attenuation with US-05 with a mash in the low 150s or high 140s isn't uncommon. If you want to keep using dry yeast, then recipe modifications might be in order, but the modification wouldn't be less grain. The thing you would want to do is sub out some base malt for something less fermentable and with more dextrins. One easy way to do this is blend some of your base malt with one of the many "Mild" malts available.
If you truly want to find the cause, then you need the manipulate just one of these variables until you fix the issue. I would probably choose the mash temperature (with new thermometer).