Why are my Annabelles going yellow?

[NotEnoughMud]

I planted a row of Annabelle hydrangeas along the front of my house (east facing) last year. This year the leaves of 3 pf them (not adjacent) have gone completely yellow. They are still flowing, but not looking happy! Is this a lack of water?

They probably have chlorosis. Some sequestered iron should sort them out, you can buy it in garden centres.