sun all day or some shade later?

I don't see the slope as a problem, it looks more like a raised bed. it looks like I can see the top of the root balls. I have the impression that the Hydrangeas were dropped into your bed right out of the containers without any breaking up of the ball to encourage growth into your native soil. If this is true you have a barrier to water and nutrients getting to the root ball. I would dig it back up and cut several vertical slices at least two inches deep into the root ball. Any root mass at the bottom of the ball, I would slice off, up to about two inches. The top of the root ball I would breakup with my hands to allow water and air to enter. Al