Is it planted in full sun? Panicle hydrangeas bloom at their best in full sun. When they receive less than full sun, they may bloom less or the color changes may last longer. More sun exposure tends to make the color changes occur faster; shadier conditions make them last a little longer. Has it been cloudier than normal? Is it getting more shade? Are there forest fires sending smoke to your area? Extremely hot temperatures may prematurely brown out the blooms instead. Hotter than normal weather can make them become a little dormant so aim to keep the soil mulched past the drip line and keep it evenly moist at a depth of 4 inches.

Full sun, none of those other factors. Same as it all was the first two years when the color changed properly though the plant was smaller.