The plants looked healthy so I planted them in an area of the garden that had been grass, which I dug up to make a flower bed. The plants are gradually turning yellow and dying. I have to say I live in low lying area and we have had torrential rainfall these last few weeks.
Wallflowers are quite drought tolerant so I suspect the heavy rain is the cause of the yellowing and dying plants. There is little to do except hope they can dry out and not succumb to root rot.
https://www.gardeningknowhow.com/ornamental/flowers/wallflower/growing-wallflower-plants.htm