Depending on how deep the shade actually gets, that sounds like a good place for them. Since you've added the info that they're small, I'm wondering if they need extra nutrients. Top-dressing around the plants with lots of compost works well over time as there's less chance of chemical burn to roots, so I would try that first. I don't use chemical fertilizers, so I have little advice to share on those.
Perhaps one or more of these hydrangea growers can offer more advice.
@bonitin
@foraygardengirl
@pirl
