What is wrong with my plants?

Hello everyone,

My plants are now three weeks old. About a week ago, I repotted them for the first time into pre-fertilized soil. Two of the plants look great, no problems. One, however, looks somewhat sick. Does anyone have any idea what it could be? Is it still the stress of repotting? It can't really be a nutrient deficiency, since, as I said, I repotted them into high-quality potting soil just a week ago. It can't really be the problem with watering either; I watered them once after repotting and then not again.

For comparison, one of the other plants:

Thanks !