you might want to use a root growth nutrient to promote healthy roots, but it sounds to me that you need more air flow through the soil. you can do this by adding peralite or vermaulite, when you transplant. i also had a problem with seedlings when they wouldnt grow and i found out it was cause my medium temp. was too cold. if your soil is cold just make shure your room temp is average and make shure your water is at room temp. when you water them.