What temp do you normally mash and what about the water treatments used? The correct ph can make a difference. I assume your crush is good too but there are so many things that go into efficiency that it's hard to look at just one. I've even read that some of the problems others have had in the past came down to less modified malts though that's not as likely as in years past.I’m only getting 68-70% as well. I’ve been stirring every 15 mins for a one hour mash. First time I did this I hit my numbers dead on. Since then it’s dropped each time (2 brews).
Not sure how others are getting 78-80% with this setup. [emoji2373]
And, Dave himself has mentioned that the average user gets about 70% efficiency with BIAB.