Are your hydrangeas in sun or shade? Did they grow in dry soil last summer? Is your soil clay or very sandy? Stunted growth can be caused by root damage, root rot or very poor drainage and soil of low fertility. Perhaps you could compare the locations of the plant that is really stunted. You can try adding compost to the top of the soil but if your soil is poor you may want to grow these hydrangeas in large pots or dig them up, improve the soil with compost and check the drainage and replant. Perhaps if you sent a photo I could better help to solve this mystery - hydrangeas are usually easy to grow shrubs. Keep growing, Marianne Binetti