OK, I haven't done BIAB yet, but it should follow the chart below for efficiency.
There are 4 things that will effect efficiency in no particular order...
- Grist make up... do you have a lot of unmalted adjuncts? Does your malt have the ability to convert the long chain starches from the adjuncts?
- Temperature... if your thermometer is way off you'll see efficiency problems. This has happened to me where I was mashing at 170 because my thermometer was garbage. If you are 145 - 164 you'll still get conversion but it might take longer.
- Improper measurement... this is my guess as to the biggest problem in homebrewing. Our measurement devices especially for measuring liquids in larger volumes are crappy at best. We're probably +/-0.25 gal at best and that can account for a point or two. Unless you weigh your water, or have an uber expensive flow meter, prior to mashing you won't know very accurately what you start with.
- Crush. If your crush is really bad (as in hardly crushed at all), which can happen especially with dorks messing with the mill at the homebrew shop, you won't be able to extract the sugars properly. I'm not arguing or advocating for any size, but if you can't see the starchy inside, then you aren't milling fine enough. I find this is rarely the case, but it's worth looking at.
When you combine these things the errors multiply and so will your final OG and those efficiency calculations go to crap.
So, my best advice would be to fastidiously measure your grist weight, water weight, use a calibrated thermometer, and take notes on your next batch.