Less water if i had to guess. Your seedling shouldn't have any deficiencies at this stage as it has all it needs within itself for the first 14 days.
Seedlings pop up to about where yours sits and then they perceivably stop growing. They haven't stopped growing. The growth is happening beneath the soil. It is establishing a root zone in preparation for nutrient uptake and above soil growth.
Just relax and make sure your environment is optimal, and you don't over water your seedling.