Welcome to NGA.
Are all the leaves yellow or just the older ones? You may not be doing anything wrong as it's common for older leaves to go yellow. I have several hydrangea trees and some do get the yellow leaves near the bottom while Pink Diamond, Tardiva and Limelight don't. Quick Fire is afflicted more often but only at the bottom = older leaves naturally die first. It doesn't do any harm to them and can be removed without any problems.
