From Book 2: Episode 9: The Guide
[Pokes Tenzin with a long stick, causing him to twitch; impatiently.] You in the Spirit World yet?
No, I am not, [Airbends the stick away; annoyed.] and stop bugging me. [Bumi folds his arms in annoyance.]
It's okay, Tenzin. Maybe you weren't meant to guide Korra into the Spirit World.
[Obstinately.] No! Just give me some time.
[Irritated.] Stop being so stubborn. It's not your destiny. I think Jinora was meant to guide the Avatar.
[Gets up and turns.] Jinora will not enter the Spirit World! She's not ready for the dangers of the other side, but I am. I've spent years training, studying, and mastering everything there is to know about the Spirit World.
Tenzin, we're running out of time.
If everyone could just be quiet and let me [Turns.] focus. [Looks down to see his daughter.]
Aunt Kya is right. I can guide Korra into the Spirit World. I'll be all right.
[Looks at his siblings, who give assuring looks.] Perhaps I will never have the connections with spirits like I always wanted... Like my father wanted me to have.
This is an instance of doing everything one can to be prepared academically, but not having the innate abilities or skill. Tenzin has studied everything available, but just doesn't have the connection to the spirits that Aang, Jinora, Korra or even Unalaq have.
Like the ability to bend, this seems to be something that one cannot just learn without having the ability beforehand.