Fissidens can be a funny thing sometimes.. takes an age to get going, then all of a sudden it can grow at a silly pace, especially after a good trim. As others have said, it loves a nice yellow drop checker and good light, but will tolerate many conditions.
Are you sure you have Fissidens Fontanus? I've seen some eBay sellers listing other Fissidens species as Fontanus, and several of these don't enjoy life under water...
Where is the tank is it? How deep is your tank? Does it get any shade??
I have Fissidens Fontanus on my Rio 125 and I have it on a piece of wood that is about 10cm from the surface and it grows very fast, on my low tech shrimp tank is grows very very very slow. It took about a year to grow 2cm.
If you have it in a deep tank attach it to a bit of wood near the surface.
tank is in living room naxt to a window but there is no sun light
is a bit deep about 55cm but first I keep a moss 15-18cm below water surface without any shade now I moved them down about 30 cm below water surface again no shade will see if this help if not I move them back