If you aren't seeing a Daily Patient Case question in your Today's To-Do's recommendations, then you've ether already exhausted all of the cases associated with the topics you've marked.
You can mark new topics via the Study Something Specific page (the blue "Manage Marks" button allows you to mark topics in bulk), and also via the "Learn" task from your Today's To-Do's page: https://med.firecracker.me/app#/tree/todos/learn?search=