Hi Guido,
Thanks for your post. Could you please right-click on the white space of Model Explorer and select Filter Model Element, then check whether you disable showing the elements on the Model Explorer?
If you want to show specific model element type, please check the type and click OK.
About only two models “Design Model” and “Deployment Model” are available when you right-click on the project node, once you create that type of Model, the type will not be available, i.e. if you created Analysis Model, you will not see “Analysis Model” available for selection again.
So I believe you created Analysis Model, Implementation Model and Deployment Model in your project, so only “Design Model” and “Deployment Model” are available for selection now - maybe checking the filtered elements will help you to figure out the answer 
Hope this helps. If there is any further inquiry, please do not hesitate to ask.
Best regards,
Lilian Wong
