Hi @Kate_Wardwell,
I’m sorry to say that the subtasks aren’t technically considered to be part of the project. This means that the rules don’t notice them being added.
To manually add subtasks to the project you can do so with [tab]+[p], but that makes the subtask appear in the project twice. Once at the level of a subtask, and once at the level of a task. As soon as they are in the project, they can be ‘seen’ by the project’s rules.
Another suggestion to make the collaborators visible is to make the native collaborators field visible:
To have the subtasks automatically be added to the project I recommend voting for this feature:
And here is some more information on how to work with subtasks: