You might want a little more temperature control with your mash...166 is pretty high. The enzymes denature at 170 (part of the reason for mashing out to "set" the profile of the beer). No doubt, they are beginning to go when you're approach the upper 160s. The higher amylase mashes usually end around 162. Most certainly, some of the beta's were dead as a door nail at this temperature.
I'm guessing this is either crush problems (which is actually by far the most common problem on this front), or the mash. I'd be shocked if that much gravity was locked in the bag. You'll get a little more from a nice hot mash-out, but I'd also bet not enough to make up your difference. I only point this out because I have BIAB'ed for years and my numbers rarely depend THAT much on methods of sparge, presence/absence of mashout, or method of squeezing the bag.