Mashing Help Please (efficiency)

For some strange reason I cannot get a 75% efficiency, my best close to 70%. I have a square cooler box mash tun. My last two was 50 and 60%. I am doing batch sparging. My grains are milled by a brewer, so I think that is not the problem. My last mashing was done at 66C (150F) constantly for an hour. So my temps are on the dot.

My last order was a bag of 18kgs (40lbs) pale malt. So maybe i am scooping more husks than grains?

Help please! I cannot understand why it is so bad!!!

 

How big are your beers? High gravity brews tend to have lower efficiencies. Have you checked your thermometer(s)?

 

What about mash ph? The enzymes like certain ranges!

 

I'd be concerned about the quality of the crush. When I get pre-crushed grains, they aren't crushed very well & I have to do a second crush. I git my own grain mill & crush'em myself now. With batch sparging I get higher OG's all across the board.