Hi Andy,
What seems to happen is that you are clicking one a screen file in Project Explorer view using a single click which is not enough to switch to the editor for the selected screen file. And so you keep editing the same screen file.
You need to double-click the screen file to switch the editor to it. You can also make the single-click to work but you need to enable the "Link with Editor" option in the Project Explorer view's toolbar. It's the icon with 2 horizontal yellow arrows. Please see the section on Project Explorer view in the user guide for more details:
https://wireframesketcher.com/help/help.html#user_interface/project_explorer