I don't know pin oak, but I do find it mentioned in lists of trees unsuitable for underplanting with rhododendron and azalea....it's on the list as an unsuitable associate, reasons for that grouping of trees are: too dense or fiberous rooted.

Have you dug around theplanting area under your tree to see what, if any, root competition there might be? Are the lower branches drooping in form so that they exclude too much light?

Sometimes these larger trees can really rob the soil of nutrients and water more than we realize...do you mulch with compost or organic plant remains?

I'm growing many oriental hybrids, foetidus, macrantha in my acidic clay. About once every 12 - 18 months, I push a stick of plain blackboard chalk down in next to each plant for a slow release of lime (three sticks for a large clump). Seems to work very well, and I don't have to be concerned about changing the PH of an entire planting area and making the acid lovers unhappy.

Hope you have them in a raised bed, because they require good drainage and bright light. They are also tolerant of drought conditions, once established. I grow them under many types of trees, including Oaks and they do extremely well there. They will grow, but not bloom well, under low growing evergreen trees or shrubs. Soil ph does not seem to matter much for the Lenten Roses, but the Christmas Rose, H. niger must have a more alkaline soil to survive.
Check the soil around your plants, if it stays constantly wet, you will need to raise them up so that the roots are in moist, but not soggy, soil. Also, hold off watering them frequently during the summer. Sometimes(like last Sept) some of mine go for a month or more without water and bloom on schedule in the winter.
Rb