Brewhouse efficiency takes into account the entire process from volumes to gravities so if your volumes are off your total efficiency will suffer.
Example: you mash and hit your pre-boil volume and gravity say at 85%, you're good, but then through the boil and chill you find your OG off and volume either high or low your brewhouse efficiency will be off of expected %.
If you have used the same mill and checked gap and have the same crush and mash process and water source then your mash efficiency should be consistent, if not then as mentioned start with the crush and pH, otherwise evaluate your post boil process and volumes/gravities