It is not that common to have serious problems getting phlox to bloom. So, if your phlox is not blossoming, there is probably either something wrong with the plant or how you are attempting to grow it. Check to see if your plant shows signs of powdery mildew. This can stop phlox from blossoming. Another possibility is that you planted the phlox where it is not getting enough sunlight. Fewer than six hours could cause it not to blossom. Another issue is over-fertilizing with nitrogen. Finally, if your phlox is well-established and used to bloom in the past, it is possible that it is simply time to divide it. If it has been a few years, dig it up and do so. You may find that the smaller plants blossom fine the next year.