What is your water temp? I had several mosses and they all looked like crap or died when I had it at 78ish degrees. I dropped the tank to 70-72 degrees and all my plants look amazing and the mosses are growing great.
strange it's one of very few plants im able to grow well. Java moss is really hardy. I have it in my tank at 26 degree celcius and it's flourishing. One thing i have noticed is that although alot of websites list it as a low plant mine seems to stop growth when that happens although it doesn't die and starts growing again as soon as adequate light is given. Other problems i had with mine was alot of dirt getting trapped in the java moss.