Korra is a reincarnation of Aang and Roku and etc. That means she is literally the same spirit, just residing in a new body with a new personality and stuff. So it doesn't matter if Korra can 'connect' with those earlier versions of herself or not, she is still their reincarnation, and her reincarnations will be as well, by definition.
But yes, I think plushu is correct in regards to Korra's not being able to connect any further back than Korra herself.
But I hate the idea of Raava being the one responsible for that, so I'd be fine with a retcon that says there's still a way to make the connection.
But I like what the loss means for Korra and I think it opens up more storytelling if there isn't a Ghost Mentor always able to just reveal helpful information to the current Avatar.
So I think I just want AtLA prequels for the rest of my life so I can avoid the issue.
But even after what happened in season 2? Raava was destroyed and later reborn, it is still the same spirit ?
I guess its the same spirit, but it gave me the idea it was a different spirit, fortunately thats not true.
I was really, really sad and pissed when it happened. Because I was so excited to see Aang being Korra's teacher in a way, like Roku was to him. It still makes me sad, but it is what it is