Looks like lack of adequate water. I know you say they are planted close by but soil conditions can differ radically even inches away and therefore drainage or water penetration. Dig down and examine the root zone - should be evenly moist (if TOO moist, over-watering could be the issue - symptoms are very similar). If you are in the part of the country that is experiencing the heat wave and drough, even a little bit more sun can cause the plants to dry out and wilt.

In your pic of the "happy hydrangea" is that two plants side by side? If so, I'd seriously consider relocating one. A typical oakleaf hydrangea gets to be a BIG plant, more so in width than in height. 6' tall and 8' wide is not uncommon :-)

Thanks for your advice - yes I will try to take a look at the root system to see what's going on. I live in VA, so yes, it has been VERY hot over here, several days in the upper 90s consecutively.

Admittedly, my watering pattern has been a bit off-and-on, i.e. going a few days without watering at all and then giving them quite a good amount of water. Although I've tried to ALWAYS water them in the morning when I know it will be a very hot day. I will try to be a little more consistent and water more often while giving them less water each time. Hopefully that will help too.

The plants are maybe 6-7 feet apart and about 4 feet from the house, so I think they are pretty well spaced out and have room to grow, at least for now. I hope that becomes a problem one day, that our plants are too big and healthy! :)

I'm not an expert and quite new myself but from what I've read its actually BETTER to water them LESS OFTEN but more deeply like you have been doing. If you change and start watering them more often but with less water the roots will not grow DEEP. You want the roots to grow deeper so that the shrub (and this works with grass too ) will be more drought tolerant. If the roots are deep, they will have access to more water during drought.
Also, oakleaf hydrangeas have roots that can EASILY be damaged by watering too frequently. That's why they say not to place them in spots that get standing water even for short periods of time.
Allow the shrub's root zone to dry out slightly between watering and then water DEEPLY.
Hopefully a regular poster can back up what I'm saying but I know I have read this often while searching through old threads here at Gardenweb.
Good luck!

The heat and drought has been horrible here in south central Illinois (100+ temps continuous for weeks on end and no measurable rain since Memorial Day). After some experimentation, personally I have been having better luck watering everything in the evening. I'm guessing that evening watering gives the plant longer to have the water in their cells before it evaporates. I try not to get the stems and leaves wet in fear it will rot.

I agree with the watering deeply theory. Especially if I am unable to water for a few days, I give my hydrangea several long drinks by giving them water then watering other stuff and returning to the hydrangea again. Maybe a "soaker" hose will help you with your watering and also control runoff. Also, mulch might help you with offsetting evaporation.

hth,
Deb

Have you examined the rootball of the unhappy ones? I find often with my shrubs that if one or two are not doing well in a bunch planted in the same environment, then I didn't do a great job of soaking and working out the roots so they wouldn't be caught within the rootball. That way they have a place to grow and the watering you are doing it is reaching them. On a shady day later in the evening if I were you I would dig the unhappy ones up and make sure the rootball is free and growing out and that the roots are not trapped in each other. If the roots still look like they did coming out of the pot then I would cut the outer roots and attempt to loosen the rootball.