I've had pulsing xenias in my tank for almost a year. Recently I had to frag and send some of the little critters (6 frags) to the LFS to find new homes.
What I've found with these guys, is that they don't really like a whole lot of flow...they like a little bit, but not that much. How much flow do you have them in, and what kind of lighting do you have?
Once you get them situated just right, these critters spread like crazy. I had one that was just starting to spread onto one of my big rocks from the frag rock it was on...I now have a teeny-tiny xenia growing where I had to pull the frag off of...crazy...
Lisa :happyfish