I had a couple finish clean and quick, but my next one to be done was showing a high FG. I've been trying to think what I did different and I believe I know what it is.
On this high FG beer I remember setting the fermentor into the temp control freezer before aeration with my mix stir.
So, if 5 gallons of 1.003 will produce about 2 volumes... 1.030 would produce 20 volumes. 20 x 5 is 100 gallons of CO2 gas. More than enough to fill a freezer. Three fermentors would clearly be enough to push all of the air out. Now you have to open the lid to set the new fermentor in, but very little of the CO2 will exchange with fresh air because it's cold and won't all spill out. So when I was aerating with it in that CO2 rich environment I'll bet I wasn't aerating as well as I thought.
Note, if aerating with a mix stir do it in fresh air, not the freezer.