If your elements are not connected to the final composite with a display module attached you will not see them. You were perhaps in “Display All” mode previously where you can view elements both attached to the network and not.
You need to have the display toolbar pulldown item set to the display module name of the network you are building. Usually being the first and sometimes only display module, it will simply be named “Display”. You can add other display modules at key composite nodes to spot-check effects at various parts in the network (using the render view).