At the beginning of warm weather I purchase some wave petunias. At first they did very well, but they started to become wilted & died. At first I thought maybe there was something in the soil, so I bought some more petunias & took all the old dirt out of the window box & put in new soil. Again, some of the petunias wilted & died. Three of them still look healthy.

What could cause a very healthy petunia to die?