If the velocity has gone down and accuracy is out the window, I would look at the breech seal fist, as Tony suggested and then the muzzle brake.
Something is slowing the pellets and something is deflecting them. It may be the same something, or it may not.
Also, just because the brake is screwed up tight, it doesn't mean it's on straight, it could have taken a slight knock at the range or even going into / coming out of the gun bag. Loosen the fixing screw(s), snug the brake onto the barrel nice and square, then tighten everything up again.
"I'm your huckleberry, that's just my game"