Answer: I believe you've answered your own question! It sounds like the seedlings are potbound and in need of nutrients. Once you repot them, put them under a grow light or in partial sun for a few days before putting them in full sun. This should do the trick!
Ideally, you should transplant your seedlings before the pots are packed with roots, and fertilize them with half-strength fertilizer as soon as the first "true" leaves appear. The first "leaves" that appear after germination are the leaves that were inside the seed. All the leaves that grow after that are "true" leaves. Enjoy!
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