why do the leaves keep falling off my garden red robin

Question

It is a photinia. I have checked all the normal reasons (i.e. wet/dry soil, sunlight, seasonal temperature, drainage, clay soil, etc.) but all are ok.

Answer 1

If you're satisfied that the culture is on point, then the problem must be pest or pathogens. Here's an article about a very common photinia disease: https://www.gardeningknowhow.com/ornamental/shrubs/photinia/photinia-leaf-spot.htm
To find out what kind of a problem you have, take some leaf, branch, and soil samples to the nearest Extension Servcie for analysis and recommendation of treatment. This link will help you locate one: https://www.gardeningknowhow.com/extension-search/