leaves falling from my Red robin

I have two Photinia Red Robin, looking smart and growing nicely but shedding an awful lot of leaves. I have one in the front (east facing) and one in the back (west facing) and clay soil.. There are also spots on the leaves. There is plenty of new red growth and other than that they look fine. Is there anything I can do to prevent or slow down the leaves falling? This part of the garden looks like autumn.