Answer: Based on your description I am not certain what is happening but I would suspect a root problem. It may be that the plant is still just not yet fully rooted so is unable to support the full growth of foliage and blooms it has developed (if you find it wilted in the afternoon but it recovers without watering once the sun goes down and is not wilted the next morning, then it is heat stress rather than strictly a lack of water), or it may not have rooted out into the surrounding soil the way it should have.
This can sometimes happen in clay soil when the amended soil is contained in a smooth sided hole, causing the roots to continue growing in a circle as though still inside a pot. You might want to gently dig down and see if this is what happened. If so, you could lift the plant and slice the roots as one would for a potbound plant and then replant. You could do this in late August or very early next spring. The other thing to check with clay soil is if your planting hole is working like a sump -- holding water in the bottom and then causing root damage due to overly wet soil. This is the result of a poorly drained location or amended soil draining faster than the surrounding soil.
The other possibility is that your watering is not thorough enough even though it is frequent. It is usually better to water less often but to water slowly and deeply so the water soaks in deep and encourages deep rooting, so a deep watering is better in the long run than a daily light sprinkling which encourages shallow rooting. The shallow rooted plant must be watered without fail since its roots are near the surface where the soil dries out quickly.
At this time of year you can't really change your watering since the plant will wilt. However, next spring you might want to try the deep watering to encourage deeper roots. The soil naturally stays moister the deeper you go. To know if you need to water, dig into the soil with your finger. If it is still damp you do not need to water yet. When you water, water slowly and let it soak in. After watering, wait about six hours or so and then dig down to see how far your water actually went; it can be surprising.
I'm sorry I can't tell you specifically why your plant is wilting but I hope this helps you trouble shoot.
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