You’ve got it, Martin!
This could also be your problem.
As a former user of a 10 gallon round Rubbermaid cooler for mashing, I routinely noticed whenever I did 10 gallon batches with gravities above 1.060 and/or with wheat malt as a significant % of my grist, that grain bed compaction would significantly slow down the lauter, especially if my grains were crushed too finely in pursuit of higher efficiencies. I also noticed how I didn’t have enough room in the MLT to add more water to correct mash temps (with additional infusions) or to increase the water/grist ratio (i.e., thin the mash).
I highly recommend evolving to the rectangular coolers to eliminate the above issues, especially grain bed compaction, as the height of the grainbed never exceeds the width (or length, depending on your point of view). For 10 gallon batches, I’m happy with my 82 quart rectangular cooler, but would consider moving up to a 100 or even a 120 quart cooler for more flexibility.
I started out with a rectangular cooler, but was having trouble getting the last of the lauter out through the braided dishwasher hose (set up with a tee). I only ever do five gallon batches of beer, so the ten gallon round cooler seems sufficient in that regard. yesterday was the most I’ve ever filled it (eight gallons) and it was at the tippy-top with the grains. I’ve only ever had two stuck mashes, one was half wheat and the other was yesterday. I only had 3/4 pound of midnight wheat yesterday, but was also experimenting with the grist and may have gone too fine. I had a LOT of little particulates I had to filter out.
I had a pretty crappy homebrew weekend overall between two batches. killed a yeast starter, had a stalled fermentation, a boilover and a stuck mash. still better than being at work, tho.
I prop the rear of the cooler up before I start the runoff. Get’s almost everything out.
I prop mine up after I’ve gotten all the runoff I can with it level.
I dont know how much this applies to batch sparging as Ive never really used the method.
However on my Brutus system(fly sparge) I tend to mash most beers at 1.25 qts per lb. Do maltier beers closer to 1 qt per lbs and most lagers and decoction mashes around 1.5-2qts per lb. Ive noticed no noticeable difference in efficiency based on the water to grist ratio. I do get slightly higher on decoction mashed lagers but I suspect that to be from the decoction and step mashing itself rather than the water to grist ratio. I generally get 85% efficiency at 1 qt per lb and 1.5 qt per lb and everywhere in between.
As you crush finer, you may need to pay attention to flow rate and husk quality. That means that you may need to start you flow slowly before increasing the runoff rate to prevent compacting the grain bed and malt conditioning to prevent your husks from shredding.
If you get 85% efficiency, then you won’t see an increase from a thinner mash because there is really no room to increase. The benefit is limited to brewers who get 75% or less, since those brewers are probably not getting complete starch conversion.