First off, I hope this is the right spot. I consider myself all grain now even though I don't have all the extra equipment.
First batch with BIAB was an IPA that ended up around 55% calculated efficiency. Luckily, I was targeting a little higher gravity so the low efficiency didn't kill me.
Second batch was a Kolsch that I also targeted high. Again, I ended up low, but this time with some mild improvement (60%). Both efficiency numbers were gotten by changing the efficiency number in the brewers friend recipe calculator until my OG matched the program.
A couple of notes about my set-up and process.
10 gallon brew pot, custom bag (fiance sewed it for me) that fits well over the edges.
Grains were sent through the mill at the homebrew store, so even though I have read that grain can be practically pulverized without any fear of tannins due to the mesh of the bag, I had 'regular' crushed grain.
Mashed between 154-150 for the IPA, and 152-146 with the Kolsch. Both were stirred periodically and the temp brought up by boiling water when it dropped. I had three different thermometers (brew pot, probe, and a digital stick) and they all pretty much were around the above temps.
Mashed for 70 mins on the IPA and 80 mins on the Kolsch.
No iodine test because I don't have any.
Small sparge of grains on IPA after I pulled the bag (~2 qts at 170 over spent grains). Kolsch grains sat over a strainer, and I added all the drippings back.
Both batches ended up at the volume I wanted, so I don't think it was an issue of adding too much water.
No pH taken during the whole thing either. No dark malt to bring acidity down, but I don't think my water would lead to higher pH (near Boston, MA).
Sorry for the length, but I wanted to be thorough. What are the top reasons I could of ended up with a low efficiency.