Can you post a picture so we can see if it is indeed what you were sold? and may I ask what you have been feeding it? as that can make a difference

Has it got enough light? If it's too shaded, that might well affect it.

If you've split the plant several times in 3 years, Sharon, each bit probably hasn't had time to recover from the shock of being split up before you've done it again! I would just leave them to gain some strength before you start to worry too much. Phlox don't need a lot of feed, so perhaps you've overfed them so that they've produced too many leaves. You need to give them a feed to encourage the flowers not the leaves, I should think. Annie (Cumbria)