Can't add items to a plate once it is sliced

[problemchild]

I created a plate with 2 items on it, then generated the gcode. I decided to add 2 more items and "prepared" again. But the preview and the actual print only had the original 2 items.

[Simplify3D]

Creator gives you extra control over what processes apply to what models on your build plate. This is necessary for a lot of the other advanced features that it offers. By default whenever you add a process it will apply to all the visible parts on the build table. However, in your case, you added 2 parts, added a process, and then added 2 more. You can change what parts are being sliced by a given process by opening your FFF settings and clicking the "Select Models" in the bottom left. Just click Select All or click an individual model to toggle if it gets included.

Let me know if that helps!