Hi , I planted three phlox plants last year in a part shade / part sun area at a distance of about 12 - 15 inches away from each other. While one of them is growing well , the other two have just one stem left, the others just fell down touching the ground , their leaves turning yellow and eventually died.

I discovered the few bottom leaves developing some white colored flecks and I sprayed sodium bicarbonate and mild soap dissolved in water on the leaves. I am also watering them every 2 days.

I do not see any improvement / growth in the two plants that seem to have some problem. Any advice would be appreciated. Thanks!