why do the leaves keep falling off my garden red robin
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.
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/