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.