There is two ways to perform a factory reset on your AT Pro. Most people tend to do a factory reset while the machine is still on. To perform a true factory reset, the machine needs to be off before you hold down the power button until it beeps to perform the reset.
I had two incidents in the last year and a half when my AT Pro was going nuts and would not clear up with a factory reset while the machine is on. When I shut down the machine and then performed the reset while the machine was off, my machine started running quiet again.
Now, I perform a factory reset before every hunt. After the factory reset, I change to the Pro Zero mode, bump the sensitivity up to full, no ground balance and GO! The AT Pro will scan as deep as it can set like this.
Before you ground balance, make a few swings over the ground or sand first to see if the AT Pro will run quiet. The purpose of the ground balance function is to cancel out the magnetic particles (iron) and grains of iron in the soil or sand to make your detecting experience more enjoyable. Whenever you cancel out anything whether using the ground balance or using the discrimination you lose overall depth.
If you don't need to ground balance ... Don't!