To get a custom field to show up on a subtask, you need to add the subtask to the project (by default, when you create a subtask, it does not get added to the project of its parent task).
Note that this does have the side effect of having the subtask show up alongside the top-level tasks in the main project view. You’ll have to decide if that’s OK with you in order to get the desired custom-field functionality.
If you’d like to have custom fields be available on subtasks without having to add them to the project, you can add your vote for that feature here. (You can see it’s a pretty popular request.)