Certified GKH Gardening Expert
A few weeks of flowering can be normal in many varieties. Deadheading the flowers should bring on another flush of bloom. If only one has flowered, then I would look at environmental issues. They are very heavy feeders, so if the soil isn't fertile, or they don't get fed often, then this could be the issue. If they do get fed, then it could be a lack of light if they are in a shaded area. To understand the requirements of these plants and begin to correct environmental issues, read this article: https://www.gardeningknowhow.com/ornamental/flowers/phlox/growing-garden-phlox.htm