I've been unable to hit my FG on my batches (I'm new to AG).
My method is a cooler, with a braid. I put in 1 1/4 quarts to gallons of water at appropriate temp, then sift in crushed grains and stir. I usually hit my temp fine.
I then rest for 60 mins.
After that 60 mins, I crack open my valve and keep the grain bed covered with hot sparge water (175-180). I use a screen that splatters the water to ensure I don't channel.
I continually adjust my valve to draw off two gallons every 10 minutes, and end up with roughly 6.5 gallons after 35 mins.
My OG is usually pretty close to the target, but a couple weeks later my FG stays about 5 points too high, once was even 10 points to high.
I'm new so I don't know if my sparge temp or rest time is doing something to the sugars that won't let them be converted by the yeast.