Q.Osteospurmum Jucundum Dying.
I have grown Osteospurmum from cuttings and they thrive in a plant pit but when I plant them into the borders, they wilt and die, almost rot. I have changed the soul and it is still happening. We had about 50 planted by a landscape gardener about 4 years ago and we only now have only about 15 original plants left.

Certified GKH Gardening Expert
It is likely a root rot issue. Changing the soil alone will not stop the issue. Treating with a fungicide will be necessary, also.
Here are some articles that will help:
https://www.gardeningknowhow.com/garden-how-to/info/using-fungicides-in-garden.htm
https://www.gardeningknowhow.com/ornamental/flowers/african-daisy/african-daisies-osteospermum.htm