Why do some new roots fail?

[snowbell]

Hello


I am growing Phalaenopsis species seedlings on my windowsill.

My Schilleriana was growing two new roots into the media, but one of them suddenly shriveled up entirely. The other keeps growing fine.

My cornu-cervi is growing many roots, but the growing tip on one shriveled away.

Is this normal, or a culture issue? Would it help if I watered the seedlings with CalMag and kelp instead of fertilizer?

Thank you!