Hi @Lucia_Araluce and apologies for the delay in responding to you!
As it stands, subtasks do not inherit the project from their parent task (you can learn more about it here), which explain why they don’t show up in your timeline unless you manually add each subtask to your Project. Unfortunately, there is currently no other way around this, but if you haven’t yet, I would recommend adding your vote to this thread from our #productfeedback category!
Hope this helps!