i bought some busy lizzie plug plants and put them in my cold frame the temperature never drops below 15 degrees but some have wilted and died the others are doing quite well cannot understand why some have died

If you have put them in a frame in the sun, it may be either too hot, too bright, of too dry.

The fungal infection of bizzie lizzies hasn't yet been eradicated for one point and Lizziebee has covered the others

Its too bad of nurseries to still be selling them as they are almost bound to die off before the summer's out.
If you really want Busy Lizzies Whyme, look for New Guinea hybrids, which don't get the wilt!

I,ve changed to osteospermum, they seem to stand the weather better, not the range of impatiens but I kept two over the winter in thr gh