I sometimes have issues with unity based visual novels. So it may be more of an incompatibility type of issue than a pure programming one. I don't know if that information can help you in any way.
Not saying it's all Unity's fault. Most likely something you coded works for others and in a few cases fails due to circumstances wich may or may not be linked to Unity either alone or in cunjunction with whatever. If i thought it was all Unity failing me i'd not have told you i hope it may help.
I only pointed my past issues with Unity for if you ever want to look down that path...
Managed to get past that point in the game anyway, and that's what currently matters to me. ^^