In the Quantum Leap series, Sam Beckett stepped into the Quantum Leap accelerator then vanished and you know the rest of the story, but in episode "The Leap Back - June 15, 1945" Sam and Al switch roles. Al becomes the leaper trapped in the past, facing mirror images that were not his own and Sam becomes the observer from the present. In the episode Al is put into a situation, which causes Sam to decide that he must leap into Al to save his life, which allowed Al to return to the present. 

My question is: 

**Why not just send the displaced person, who would be in the waiting room, back through the Quantum Leap accelerator, which would allow Sam to return to the present?**