have you added the stories to a Sprint, and did you start the sprint? I think this is required for them to show up in your modeler when you open the project. I remember having the same 'problem' when I first started.
Possibly your stories are in Sprint 1, and your Getting Started sprint is still 'running'. The modeler only shows the stories of the current active sprint.
I had the same issue but other solution, my project management (under the general settings) was set to Epics, you should change it to Mendix Stories and that solved it for me.
You probably added the stories to the backlog. In order for them to show up in the modeler, they need te be in the current sprint. You can create a sprint in the Collaborate -> Stories tab in the developer platform.