I planted this dish garden last November with fittonia, selaginella, and two unknown ferns. Everything has grown, but very slowly. I expected some height from the ferns by now, and I'm wondering if I could be doing something to help.

They are in a shallow (about 2") galvanized container with many drainage holes which I water about every three weeks, or when the mix is almost dry. I take the whole thing to the sink and let it drain freely before putting it back in its little greenhouse. I leave the lid wide open for a few hours, then keep it closed (though it's not airtight). I've fertilized weakly a couple of times this spring/summer. It's in commercial potting mix with additional small bark and perlite.

I've moved it a few times trying to get the light right - bright north window, below a south window, and now several feet away from a large south window. The fittonia got very pale in bright light. New growth came in darker when I moved it to lower light, and the selaginella seemed happier, but everything is still so short! I was envisioning tall ferns, a medium-sized mound of fittonia, and low-growing selaginella. Should I just be patient? Is there something I'm missing?