I would start with the basics. Check the water availability, soil pH and the mineral levels either with a test kit that checks for NPK or a formal/real soil test. I have also had some hydrangeas that barely grew until almost the third year (must have been busy developing the root system).

Something else that produces stunted growth is when roots grow in a circle within the pot/container. If you do not cut them prior to planting, they will tend to stay growing like that so you have to extract the plant, prune them and replant.