Grain crush size matters when using BIAB. Go with a very fine crush. Look for a corn meal consistency with lots of flour. Your OG will go way up. I usually get 85% efficiency. If you have a bunch of dimensional chunks then it is too course.
If you are crushing the grain, go with a tight setting, about the thickness of a credit card. If your local home brew supply store is crushing for you, ask for a double crush or if they can tighten up their mill. Using the typical crush size for traditional mash tun method will not get you the standard 75% efficiency because the mash thickness is typically much thinner (2.0 - 3.0 Qts/Lb or more) with BIAB.
Are you dunk sparging, no sparging, sparging? You have to take that in to account. No sparge means you need a heavier grain bill since you will be leaving sugars behind in the grain bed after the bag is removed. Dunk sparging will help wash out some of the sugars. I find a long slow sparge and letting the bag hang over the kettle for a long time to drip dry for like 20+ minutes helps get all of those sweet sugar particles into the boil. Dont be afraid to give the bag a good squeeze before moving the bag out of the hanging postion. Keep your sparge water around 165F, dont go over 170F. Squeezing wont release tannins, its a myth rooted in fiction. Large breweries have hundreds and hundreds of pounds of soaking wet grains crushing down in the mash when they drain the mash.