Answer: Some hydrangeas are sensitive to soil pH and will color accordingly. Blue flowers would be produced when the pH is below about 5.5 or 5.0, pink when it is in a range of, say, 6.0 to 6.5 with intensity increasing as the pH becomes more extremely acidic or alkaline.
Some hydrangea varieties are hardier than others, so this could account for one plant not blooming. The location could also be more exposed or windier or colder for some other reason and this could contribute to lack of bloom. The large leafed hydrangeas bloom on the previous year's wood, so any damage to it by winter weather or by ill-timed pruing will kill of flower buds.
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