I have found xenia either loves a tank or hates it. If it loves it, it takes over if it hates it then it will hang on by a thread for the longest time and then die. Give it time and maybe move it to a different flow area.
I have actually had none in a tank but all the sudden I look and it is coming out all over the place, apparently the rocks had some on them and when the conditions were right it just started growing and growing. It isn't the pulsating xenia it is another type, I am hoping to keep it under control.