soil most likely?? using earthworm castings gives your seedling a gentle feed aswell that helps with the process.
I don't know what the problem is, it gets a good 6-7 hours of sunlight a day. How much should I water it when the soil is dry?
My week olds already have their first set of true leaves, and they have been dealing with conditions worse than I'd prefer. Yours are only on their 3rd set of true leaves, should be hitting right proper veg stage by now I think.
I had a similar problem, my soil had poor drainage so the roots never really expanded. I repotted with better soil and it instantly took off. From the pic there looks to be poor drainage in yours, but I can't tell for sure. Is it in a pot or in the ground?
The plant is in the ground and I just transplanted it from a cup 5 days ago. I think the problem may have been how much water I give it.
If you started them inside it could also be your light source. I learned a very valuable lesson this spring with my lighting. I had started 3 seeds with my Veg garden seedlings under 40w floro's. After I transplanted them into larger cups I lost them to neglect. So I had to start over with 3 more seeds, this time I used a 100wMH and I was shocked at the difference in growth rate of the seedlings. I will forever use the 100wMH for my seedlings now.