Pro Ball NW notes that Jason Vargas of the Seattle Mariners doesn't feel like he's pitching badly. That's quite possible. Sometimes batters simply hit good pitches. With Jason, however, that's not the case.
Through July 20th, Jason sported a 3.94 ERA, with 125 hits allowed in 130 1/3 innings. He was also keeping his change up down.
In that time, batters hit .270/.332/.456 against the fastball but .212/.268/.297 against the change. The pitch is designed to fool batters, and it was working fine.
Starting on July 25th, Vargas holds a 6.46 ERA in seven starts, allowing 53 hits in 39 innings. Note the difference in his change up:
Vargas puts more of his changes up in the strike zone, and more in the lower corner to the left-hand of the catcher, getting away from the area of his success. In this period, batters are hitting .333/.406/.533 against the fastball and .292/.354/.431 against the change. With his change up no longer fooling batters, his fastball becomes less effective as well. He is pitching badly.