Benches have horrible ergonomics for trying to shoot a precision rifle. By design, the body is completely canted when shooting from a bench, which the recoil of the rifle will exploit.
Without seeing your groups, its all just a guessing game of what's going wrong.
- Are you perfectly square behind the rifle? (probably not, if shooting from a bench)
- Are you consistently applying the same pressure on the cheek rest from shot to shot?
- What pressure is the firing hand applying to the grip?
- Are you pulling the trigger STRAIGHT back, and holding slightly after each shot (i.e. not slapping the trigger)
- Are you applying consistent pressure into the bipod, shot after shot? Is the bipod square to the target?
- How is your rear bag being controlled by the support hand? Are you subconsciously doing a "sympathetic squeeze" each shot? Is the bag setup correct to the rifle?
- How is your breathing control? Are you breaking the shot consistently at the same time in your breathing cycle? You aren't holding your breath, right?
I never shoot from a bench, the ergonomics are horrible. I lay prone next to them, squaring up as much as I can behind the rifle. And each shot I focus on the fundamentals.