Onion Question

I’m a beginner gardener/homesteader and all except for one container of my onions are doing well. Same soil, same amount of light, water, etc. The onions in this container or significantly thinner than all of my others that have started growing. Also, the one in the middle is so weak that it flopped over and the tip is turning yellow/ brown.
I don’t know how to fix it and need help/advice.