I have had tall garden phlox for 4 years and it grows very tall but has never flowered. There doesn't appear to be any powdery mildew and the plant is healthy. It gets about 6 hours of sun daily. Why doesn't it flower?
Why Doesn’t My Tall Garden Phlox Flower?
Certified GKH Gardening Expert
I would guess that the nitrogen to phosphorus ratio is off. The soil may have an abundance of nitrogen, which would result in nice, healthy green growth but little to no blooms. Without the right amount of phosphorus, it will not bloom well. Bone meal is a great way to add phosphorus.