Star Wars has subtly explained why Rey couldn't sense the old Jedi before Star Wars: The Rise of Skywalker. The early scenes of Star Wars: The Rise of Skywalker are among the most curious in the entire film. Rey has been training as a Jedi under Leia Organa, who has encouraged her to use the Force to hear the voices of the Jedi who had come before her. For some reason, though, the Jedi are absolutely silent.
Rae Carson's novelization of Star Wars: The Rise of Skywalker unwittingly made their silence seem far stranger. Carson revealed the Force Ghosts had been training Leia for years; she had occasionally heard the voice of Obi-Wan Kenobi, and even more rarely Master Yoda herself. After his death in Star Wars: The Last Jedi, Luke's Force Ghost had been appearing to her to guide her too, although Leia had been stubbornly refusing to heed his advice that the time had come for her to become one with the Force. So why didn't the Force Ghosts manifest to Rey?
The junior novelization of Star Wars: The Rise of Skywalker implicitly offers an answer. It draws a subtle parallel between Palpatine's seduction of Anakin Skywalker in the prequel trilogy with his attempt to draw Rey to the dark side; the Emperor is the one responsible for casting dark dreams and dark visions into her mind. "She still had other dreams, however," the book notes, "and these were more like waking nightmares. They showed her dark places she did not know, strange figures she did not recognize, and ominous events she did not understand." Palpatine's goal appears to have been to wear down Rey's will, to persuade her she was destined to fall to the dark side. He came within a hair's breadth of success.
This implicitly suggests another parallel between the prequels and Star Wars: The Rise of Skywalker. At the beginning of the Clone Wars, Darth Sidious somehow used the dark side to weaken the Jedi. "Our ability to use the Force has been diminished," Mace Windu observed in Star Wars: Episode II - Attack of the Clones. The technique has never been explained, but in the old Expanded Universe it was compared to a veil Palpatine had somehow set up, blocking the Jedi from accessing the Force for wisdom and insight. Presumably he did something similar to Rey in order to ensure she could not access any sources of light and hope; indeed, her mounting frustration with the Jedi of old would have probably pleased him, because it had the potential to hasten her fall.
This perfectly aligns with the overarching narrative of Star Wars: The Rise of Skywalker. Luke Skywalker only appeared to Rey when she was on the planet Ahch-To, a world rich in the light side of the Force, where Palpatine would have struggled to hold the veil in place. Furthermore, in the third act, Palpatine forgot about Rey after he had drawn the power of the Force Dyad into himself; he considered her no longer worthy of his notice. The veil was lifted, and the Jedi of old were able to speak to Rey at last, offering her the words of wisdom and encouragement she needed to hear. And so Rey rose to her feet again, and confronted the Emperor who had thought her defeated. "And I am all the Jedi," Rey told him as she advanced in spite of his storm of Force Lightning, and perhaps at that moment - just before he was consumed by the dark side energy blasted back at him - Palpatine realized his mistake.