My theory is Qui Gon would've appeared as a force ghost to Yoda and Kenobi only. The dying Jedi chooses who they will "haunt", not vice versa. "An old friend has learned the path to immortality. One who has returned from the netherworld of the Force... Your old master" So once Luke dies, he can choose to haunt an existing Jedi, but that Jedi won't see Yoda and Kenobi. Come to think of it, Anakin was never trained of the "Path to Immortality", how did he come back as ghost?
Qui Gon indeed was made for the prequel trilogy, so at the time of ROTJ he didn't exist to manifest. However he did show himself to Anakin and Obi-Wan in the Clone Wars cartoon. So when Luke triumphed, he could have shown up to be with his friends(Yoda, Obi-Wan), and show support for his student(Anakin), and his son.