Hey
@TheFunks!
I don't think it's a nitrogen problem. I would consider checking the watering schedule, they look a bit overwatered to me.
You should check how much water do you give to them. It should be around 15%-20% of the capacity of the pot each 2-3 days. As I saw on your diary, you are giving them 1.89L (wich is not bad) but just not everyday. ΒΏAre you sure that pot is dry when you water them?
If you think the problem is related with nutrients, you could check the output water of each pot (when you water them). You can check the EC input and output and know if you are having sallt acumulation problems. As you don't have EC data I will assume you can't check it. I would recommend you to water them just with water and check the output till it goes totally transparent/clean. That should help salt acumulation or overfeeding problems.
Good luck! π