Many seedling problems come down to watering. It commonly seems to be over watering whenever someone just whatever's every "x" days. What works best for me with seedlings is a small medicine dropper that I can get 3, 4, 5 etc. ml of h2o out of at a time and when I see the soil is dry I water again. Many people mist their seedlings, I've never done that so I can't properly explain, even though it seems pretty straight forward.