Sue - I grow mine in cups, so if I bump the cup or accidentally brush across the leaves of the seedling when they first sprout, the root(s) have not had a chance to get a good hold down into the soil and they sometimes fall over. Planting them back into the soil is a real challenge without damaging them. (At least for me.)
The biggest issue I have found is watering them. What works best for me is to make sure you have enough holes in the bottom of the cup/pot and water them from the bottom. I put my cups/pots in a small plastic bin and add water to the bin so the soil (in the cup/pots) can soak up the water through the holes in the bottom of the cup/pot. This also helps the roots grow deeper instead of growing near the surface of the soil if watering from the top of the cup/pot. I always add hydrogen peroxide to the water and then pour off any excess water after about an hour of the cups soaking in the plastic bin, to prevent damping off.
I totally agree with Sue ... If the potting mix bag has been opened or not completely sealed, then contamination is possible. Gnat larvae and damping off fungus are the two biggest killers of seedlings.
Your seedling will need light. Sunlight coming through a window is good, but remember that glass windows can magnify the sunlight coming through it and can get too hot which may burn/fry you seedling. Make sure that your plant isn't right up against the window. Mine are right next to my glass window, so I put a sheer curtain panel between my window and the seedlings on the windowsill. That seems to diffuse the heat and magnified light coming through the window.
