3 year old standard ‘Red Robin’ is very sparse and not thick and healthy looking

Question

Losing quite a lot of leaves

Answer 1

The best location for this shrub is one that has full sun or partial shade. Plant it in well-drained soil in locations that get good air movement.
Avoid wet soils, as this can lead to root rot.
Water the plant once a week at the base during dry periods, and avoid getting the leaves wet.
These articles will help refresh you on care requirements.

https://www.gardeningknowhow.com/ornamental/shrubs/photinia/photinia-leaf-spot.htm
https://www.gardeningknowhow.com/ornamental/shrubs/photinia/growing-red-tip-photinia.htm
https://www.gardeningknowhow.com/ornamental/shrubs/photinia/red-tip-photinia-fertilizer.htm