People ask good questions to which I sometimes do not have an answer. A common one is, when you plant 2 or 3 or 5 of the same plant in an area and only one dies, what happened? I have an example in my garden now. I have three Greek columnar basils planted along the edge in one square of our four square garden. Greek basil is a very nice, upright, bushy form that grows into a small shrub in a season. My line makes a basil 'hedge' in the garden. The plant has great flavor so it's useful in the kitchen, and it has an upright habit that requires no pinching to branch out and it sets few flowers. The two end plants are fine and the middle one is very wilted. That's a pretty easy answer - with the very dry weather, the end plants have taken the soil moisture away from the middle one and once it did rain, the middle one perked Up again, so that was a good guess.
But often, the plants are the same sort, they‘re in the same type of soil and receiving the same cultural conditions. If it's not a disease or insect problem, a soil analysis may indicate a reason. But I generally tell folks, plants are like people - some are stronger than others. Being the same species does not mean the plants are exactly the Same.
If you're interested in drying hydrangea flowers, now is prime time. They should be starting to dry on the stem - not papery dry, but you feel them, you can notice some moisture is leaving the bloom. I cut mine, strip off the leaves, stand them in a vase and put the vase in a closet (or anywhere away from light.) Some people recommend putting a little water in the vase, but I never found this necessary. They dry in about a week or ten days if it‘s not humid. Newer blooms which replaced flowers I out earlier in the season are still full of moisture, so I'll check them in a couple weeks.
These later blooms really keep their blue color nicely.