Something might be killing your actor as soon as it is created.
It might be falling offscreen due to gravity before you can see it.
Something might be moving your actor offscreen when your scene loads.
Something might be hiding your actors.
If that's a behavior, you forgot to attach it to your scene.
You can use the print block in any of the actors that are suppose to be created, and check in the log viewer if anything is printing. That will show if your actor is killed or if it was ever created. You can also test that with a scene that has nothing in it and 0 gravity.