Photinia is susceptible to fungal diseases. The most common is leaf spot, which show up as bright red spots on newer leaves that spread and kill a leaf. It is too late to treat it for this year. Clear away fallen leaves. If you choose, you can put it on a spray program next spring.
The stunting could be due to any number of problems, including root girdling and root knot nematodes. Take an affected branch to the county office of the Cooperative Extension Service; they can send it to a plant pathology lab if need be. You can also try taking a sample to a locally owned garden center.