It's gotta be your media or your water. If other seedlings are doin OK, it's a head scratcher ! Could your water or container be contaminated with something ? Seedlings usually die because of 3 reasons. 1. too wet 2. overfert 3. fungal prob "damping off." Root damage when transplanting can do it too, but that shouldn't be the case. Like Miggs said, environmental issues will kill a seedling, but that is rare ! Again, I'm not a fan of putting seedlings in a big pot early !!! GL