Tim Lincecum of the San Francisco Giants pitched poorly so far in June. He continues to strike out a ton of batters, but it's not leading to a low batting average allowed. The difference between Tim now and the Lincecum of March through May is the location of his change up.
Notice that Tim threw the pitch out of the strike zone. Coming in eight miles an hour slower than his fastball, hitters expected a pitch in the zone and would swing and miss.
Batters could not square up the pitch as they went 0 for 31 against the change through the end of May with 15 strikeouts.
Once June came around, Lincecum started leaving the pitch relatively up:
Tim appears to be throwing the pitch the same way, as an examination of the spin showed no obvious changes. With the pitch up in the strike zone, however, batters are making contact.
Batters are recognizing this pitch better, and the ball more likely ends up in a better spot to hit. Hitters are now 5 for 14 against the pitch, .357 with four strikeouts. Lincecum is no longer fooling batters with the pitch.