There is a section in that website that answers your question on why hydrangeas fail to flower.
I have cut and pasted it for you. Hope this helps.
Why doesn't my hydrangea bloom?
There are three possibilities for lack of flowering among the hydrangea species. The first two and too much shade and improper pruning and apply to all hydrangeas, while the other and weather-related damage to flower buds and applies primarily to the bigleaf hydrangea.
While most Hydrangea species benefit from some shade, too much shade can reduce flowering. This is particularly true of panicle hydrangea, which is the one Hydrangea species that grows well in full sun. If you have a hydrangea that used to bloom well but now flowers only sparsely, evaluate whether the growth of nearby trees has reduced the amount of light that reaches the hydrangea. If so, you may want to consider moving the hydrangea to a sunnier location.
Improper pruning can also reduce flowering in Hydrangea. Since bigleaf and oakleaf hydrangeas flower on previous yearÃ¢â‚¬â„¢s growth, potential flowers buds would be removed if the plants were pruned in fall, winter or spring. Panicle and smooth hydrangea flower on this yearÃ¢â‚¬â„¢s growth, so pruning them in early summer would reduce or eliminate flowering for that year.
The most common reason for lack of flowering in the bigleaf hydrangea is unfavorable weather. Most H. macrophylla cultivars flower primarily on previous yearÃ¢â‚¬â„¢s growth. Weather conditions that damage aboveground parts of the plant can reduce flowering. Damaging weather conditions include early fall freezes that occur before the plant is completely dormant, extremely low winter temperatures, and late spring freezes that occur after the plant has broken dormancy. In USDA Cold Hardiness zone 6 and warmer, which is the recommended growing area for H. macrophylla, the most common of these unfavorable weather events is late spring freezes that damage tender new growth. This is particularly true in the southeastern U.S., where "see-saw" temperatures are very common in the spring.
Bigleaf hydrangea responds quickly to warm temperatures in late winter and early spring by breaking dormancy and producing new leaves. Unfortunately, these spells of warm weather are often followed by periods in which temperatures reach well below freezing. The severity of the damage caused by these freezes depends on how many of the buds had broken dormancy. If a substantial portion of the buds on a stem were actively growing, the whole branch may die. For some cultivars, the loss of the aboveground part of the plant will completely eliminate flowering the following summer. The plant will produce new buds from the base of the stems, but stems produced from these buds will not flower in these cultivars.