Sounds like the flowers are losing moisture too fast and the cuttings then "downgrade" from nice to awful fast. . Someone posted about this eons ago and I recall a solution that was offered was to strip off the leaves and to cut the bottom of the stems at a 45 degree angle under lukewarm water. not hot water Then fill up the vase with water almost to just below the flower heads and to mist the flower heads or apply hair spray over the flower heads. An aspirin or other florist-provided chemicals will also help extend the life of all flowers. Try that or do a search for the original posting.

This is a very common phenomenon with cut hydrangeas. Typically, the reason given is while the stems are out of water (the drive home from the grocer or florist, etc.), the stem ends begin to callous over and are no longer able to take up water properly.

The best solution is to get them home ASAP and recut the stems before putting in a vase or container. I prefer to cut on an acute angle to maximize the water conducting tissue exposed or to mash the lower portion of the stem with a hammer to allow proper water uptake. The mashing method is recommended for any woody based cut plant material used in arrangements.

And keep them in a cool location and out of direct sunlight. Heat and sun can have a very debilitating effect on any cut flower.