This problem is not uncommon. The usual reason is improper pruning since it is very difficult to know whether those dead-looking stems really are dead. Some of them actually contain next year's blooms. Some hydrangeas bloom on stems formed the previous year; others form on current year growth. I suggest you not prune your shrub for 2 years until you can tell how your plant blooms. Plants also need consistent moisture. "Endless summer" and similar varieties need more sun than traditional hydrangeas which prefer morning sun or dappled, filtered light in afternoons. Use a long-acting fertilizer with the first number (N) is the same as or lower than the second number (P). Too much nitrogen stimulates leafy growth, not flowers. Several inches of compost may be sufficient. More info in these articles. https://www.gardeningknowhow.com/ornamental/shrubs/hydrangea/hydrangea-not-blooming.htm
Answered on September 12, 2018
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