Did you plunge each pot in a bucket of water to make sure the rootball was wet through before planting? If it was not wet to start with, then watered in, the root ball can dry out and even if you are watering around it, it won't absorb the water.
The best time for planting perennials is either March when the soil is warming up, or late Autumn before it gets too cold.
It may be worth lifting a couple and having a look at the rootball. If the roots are tightly bound together and the rootball is dry in spite of your watering, try following fidgetbones' advice and putting them in a bucket of water. When they are wet through, break some of the rootball apart-it may look drastic but it encourages new roots to form. It's possible that you may just have been unlucky with the timing, the sunniest May since records began, and very little rainfall (down South anyway).
I’ve got several perennials ready to plant out, as it’s been too dry I’ve kept the pots in grow bag trays with water so they keep moist. You can buy a perennial any time of the year and plant it out. If we don’t get any rain that’s been promised in the next few days, I’ll just pot them on until the weather conditions are right. I've had the same pattern of weather for a few years now, hopefully we’ll get a good soak and I’ll be set to plant out.
Gardening on the wild, windy west side of Dartmoor.
Also, if you took the plants direct from arrival and plop them outside in full sun and weather, they are going to get stressed and suffer. They've probably been in a greenhouse their entire life, then in a shipping box for a few days. The plants need to harden the cell walls before they can be put outside for the entire day/night, a process called 'hardening off'. Do an internet search, tons of videos or blogs to read.
Is your soil from a bag? Is it dark, almost black? Sun and heat are attracted to that dark soil and can cause an issue too. One time I put a bag of organic rotted manure around my strawberries that were well established, but only had time to mulch half the bed. The next afternoon the strawberries in the black soil/rotted manure area had gone limp and ended up with crisp scorched leaves.. where as the mulched ones did fine. Soil was moist, and all other conditions were identical.
I buy perennials all through the summer from the clearance rack at the GC, some almost dead and many just fine but past blooming.. and plant them our year round in the hot arid Utah summer after hardening them off or gradually exposing them to more sun over the span of a week. They usually do just fine, as long as I keep them well watered until the roots have a chance to grow. After a few weeks I reduce daily watering so roots will search farther for moisture, and just water once it's dry an inch down.
Maybe you could rig up some sort of shade cloth over the bed, temporarily? A few feet up with open sides? Horticulture fleece would work fine, hooked up to the ends of canes like a wedding tent.
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It's possible that you may just have been unlucky with the timing, the sunniest May since records began, and very little rainfall (down South anyway).
If we don’t get any rain that’s been promised in the next few days, I’ll just pot them on until the weather conditions are right.
I've had the same pattern of weather for a few years now, hopefully we’ll get a good soak and I’ll be set to plant out.