Chow_13answered grow question 2 years ago I would guess nutrient burn. Most manufactures recommendations are to strong for baby pot plants. Also you have them in 7 gal bags, But it looks to have around 2 gal of space left, So I would say you only have 5 gal of medium.
Its best to always start light and increase as you can always add more but its much harder to remove. I would re-plant them in a reduced strength mix of only 3tbsp( 2 gal dose ) mixed in to the 7 gal. And top feed when needed.
Good luck and happy growing