Clematis leaves developing reddish brown spots

Can anyone please explain to me why my clematis is developing reddish brown spots on its leaves?  It is called Dr. Ruppel and it is a new plant.  It faces West and I have shaded the roots with a small slab and fed it in the Spring, I also keep it watered in dry weather.  I have planted clematis plants before but haven't had much success, although I have a Clematis Montana and it has climbed and flowered well for many years. My soil is alkaline/neutral and I have been training it up a trellis as it has grown. What am I doing wrong please?