20-20-20 is very strong for such young plants.
Water them to the point where the water runs out the bottom with plain tap water.
Test the pH of the runoff.
Check that it is between 6.5 and 6.9.
They are a LITTLE small to be feeding yet, and the first feeding should be with a much weaker fertilizer that is a real grow fert, such as a 6-2-2 mixed to half the recommended strength.