I am wondering if it is getting too much sun. Is it sitting on a window ledge or in the middle of the room? If it is on a south or SW window ledge, it is likely too much unless you have trees shading it or a sheer curtain or something. One way to tell, is to touch the leaves when the sun is shining on them and if they are warm, it is too much. Phals don't like direct or bright light. Maybe that is why the leaf is turning yellow and flowers wilting. Without pictures, it sounds like the roots are pretty good.
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