What can stop my cone flower and black eyed susan wilting?

Hi i just need a bit of advice regarding some plants i have recently bought. The plants in question are black eyed susan (Rudbeckia hirta i think) and Echinacea purpurea. They are planted in good soil that is well drained and they get full sun. The problem i'm having is that during the day they tend to wilt and the leaves curl, almost as if they don't have enough water, the cone flowers are particularly bad. They get watered regularly and i've read that this might be the problem, are they getting too much water or is it something else? thanks for any help you can give me.