From what I've read, they like full sun and moist soil.
Any possibility one of the seedlings dried out? Are they getting any sun?

Here's my seedling from this spring..(tangled in with a morning glory), I don't let it sit in water, but never let it completely dry out either...but close. Did the leaves change yellow and drop?

I believe they just dropped without turning yellow. But I can't exactly remember now! I wonder if I should be giving them more sun