Hmmm..
I wonder if it is starving? What is your nitrate level?
I would definatly test all of your parameters. My Cheato is growing like a weed. Basically I have a shop light with a 75 watt full spectrum incandecent bulb and about 5 on my nitrates.
Maybe you need to add a small powerhead to your refuguim to cycle the water around a bit. Water just might be flowing over the top and keeping the water dead below.