One of the earliest couple of times I did BIAB, I did not stir very well after mashing in, and was surprised/dismayed to find I had way undershot my target OG. It wasn't until the next day emptying out the Igloo cooler (my mash tun) that I discovered I had big clumps of dry grain inside sizeable doughballs. Now I stir a couple of times after mash-in, just to be sure. I discovered that I can open the lid a few times without really losing much heat.
That and getting a mill to achieve a finer grind went a long ways toward sugar extraction, I get pretty close to 85%-90%. (I also routinely 3-step-mash from 149 to 153 to 158 for most beers. Not sure how much of a difference that makes realistically, but it might.)
Anyhow, don't be too dismayed. You'll get it.
Incidentally, that low OG (and resulting low alcohol) beer actually turned out great. Here I was all bummed out and debating whether or not to keep brewing, and people LOVED that beer. Sure, I wasted way too much malt to make it, but it was a pleasant mistake to learn from.