Ask Mr. Smarty Plants

Tall Garden Phlox are hybrids of Phlox paniculata (fall phlox), which is native to Missouri.  However, hybridized plants can present a problem, as you don't know exactly what their parentage is, and what attributes of the original plant were edited out and what others were added in to gain more height, different colors, longer blooming time, etc. We found an excellent website, Tall Garden Phlox for Minnesota Gardens, from University of Minnosota  Extension. We realize you are not in Minnesota, but the principles should apply. Please read the whole article but some of the things you should probably give attention to are:

The amount of sun (you already said you had full sun).

The soil being well-drained yet retaining adequate moisture; this may require incorporating a generous amount of organic material (compost, etc.) before planting. Plant grows best when it's moist.

Good air circulation to prevent mold, and watering soil, not sprinkling from above, for same reason.

Light application of balanced fertilizer as new growth emerges each Spring, again just before plants begin to flower. Not too much nitrogen, and don't use lawn fertilizer, which is high nitrogen. The high nitrogen encourages leaf development (as in grass) at the expense of flowering.