Reply in the existing thread if the new question depends on the answer already given. Briefly say what you tried, what changed, and which step is still unclear. That gives helpers continuity without making them reread the whole discussion.
Start a new thread when the problem has a different cause, product, or audience. Link the earlier discussion for context, then describe the new issue on its own. For example, “The CSV import now works, but how can I schedule it?” may deserve a new topic; “Quoting the field fixed two rows but a third still fails” belongs in the original.
Keep one clear question per reply so the next answer can be specific.
Akousa