We know that Anakin Skywalker has become a Force Ghost — at the end of Episode VI, Anakin's Ghost is a wallflower at the ewok party on the Forest Moon of Endor.
We also know that it doesn't necessarily take special training in order to commune with Force Ghosts — while Yoda mentions in Episode III that he can teach Obi-Wan to speak to Qui-Gon's Ghost, Luke can speak to Obi-Wan's with no special training.
In The Force Awakens, Kylo Ren meditates in the presence of the helmet of Darth Vader and pleads to Vader,
"Show me again the power of the darkness..."
I assume that Anakin has not actually appeared to Kylo, for if he did, he would have surely advised Kylo to return to the Light and to give up his obsession with the dark period of Anakin's life.
Given that Kylo is Force sensitive, given the stakes involved, and given the
familial connection between Anakin and Kylo,
why hasn't Anakin's Ghost appeared to Kylo Ren?