In 2007 and 2008, Jenks' best years, his fastball was 8.5 and 8.7 runs above average, in 2006, his curve was 9.4 runs above average. Last year, his fastball was 2.9 runs below average, and this year, his fastball is exactly average and his curve is .3 runs below average. The guy has lost his stuff, and that is why he is failing.
His WPA is -.38, he is below average in the clutch (obviously), he throws his fastball only 69% of the time, his curve has gone from being used 17.4% of the time in 2006, when it was rated as 9.4 runs above average, to 7.9% of the time. He has gone from a good flamethrower, to a bad pitcher.