Partial Shade is best. (This means more than 4 hours of sunlight – more like 6)
An Eastern exposure next to a building or fence is ideal.
Ensure the plant gets some sun to ensure flower production.

Water often.
If it looks wilted, it’s thirsty.
If it doesn’t get enough water in the spring (when flower heads are forming) you will have fewer and smaller blooms later.

Acidic soil produce blue blooms.
Soil with lime produce pink blooms.
You can change your soil by adding either lime or aluminum sulfate(acid).
Add Compost annually to your soil
Water retaining mulch is also helpful in keeping soil moist.
Well drained soil is a must.

Plant new plants in the spring after the last frost.
Work the soil and add compost.
Water thoroughly after planting.

Vine and Big leaf types should be pruned immediately after flowering or not at all.
Prune back to good pair of buds, the last pair if you want to dwarf the plant.
Pancile hydrangeas should be pruned in winter or very early spring and pruned HARD.

Could be caused by:
Frost damage
Late pruning
Too much shade (partial shade means more than 4 hours of sun)
Too much nitrogen