One of the hardest things about growing in a greenhouse is regulating the heat in spring. I took my greenhouse down after burning up plants on a hot spring day. Just last week it got up to 90 degrees here in OKlahoma. It should have been @65. That could be what’s happening there. If the greenhouse isn’t monitored for temp and ventilated well, you can get leaf scorch but the soil is still moist. So you water again and then create another problem. Yes growing seedlings is challenging, just like farming, every year presents a new problem. Like the other posts said it could be one or all of the issues stated above. Or it could have just gotten too hot for new leaves to withstand.