When you create a new project and add new pages they will automatically open in the workspace. However if you reopen the project then only the first page will be opened (to reduce the loading time of the project and limit the allocated memory).
To open other pages of the project you must use the Site Manager.
If you do not see also your pages then you may have added them as a child of another page. Click the ‘+’ sign in front of the page (or folder) to expand the tree.