One of my saplings don’t seem to be going as goo...

Highroller mentioned humidity not being an issue here but I can tell you from my experience it is very important in the first 3 weeks. Can mean life or death! What I do and works wonders is to cut the top of a 2litre plastic bottle and use this as a cover (dome ) for the young plant, spray the inside daily with water and this will raise the humidity of that plant to70%+ this is so Beneficial as the plant is not using its roots right now so you can prevent overwatering from just absorbsion through the leaves, hope this helps because trust me it really does! 🌱👍😁