An episode of Star Wars: The Clone Wars helps explain why Obi-Wan (Ewan McGregor) was initially unable to hear the voice of his Force ghost master Qui-Gon Jinn (Liam Neeson) in the Obi-Wan Kenobi series. Though he eventually succeeds, it took Kenobi years to even begin the process. Having been first taught to Jedi Master Yoda, who in turn told Kenobi how he could commune with his late master through the Force, Obi-Wan still struggled to hear Qui-Gon's voice, and the animated Clone Wars series holds the answer as to why.
Despite being given training from Yoda at the end of Revenge of the Sith, Obi-Wan Kenobi was unable to speak with Qui-Gon during his exile on Tatooine, watching over young Luke Skywalker while being tormented about what happened to his father Anakin, Obi-Wan's corrupted apprentice who became Darth Vader. It's not until the end of the Obi-Wan Kenobi series that Kenobi can both see and hear Qui-Gon's Force ghost. As such, it has everything to do with an inherent requirement when it comes to Force ghosts as revealed in The Clone Wars.
