Two tips for restoring ThruLines, Nov 2019.
Settings can cause a problem even when they didn't previously. This is a dynamic system, not a static one.
1) Check the parental relationships to ensure that they say, biological. If at any point they 'flipped' to unknown, ThruLines won't work. Use this article as a guide. support.ancestry.com/s/article/Fixing-Relationships-in-Trees
2) Use the Support Chat so someone can check your settings.