You have to finish the Manor quest to open all the places in Transylvania, and then make sure your Mystic skill is 7+ before asking Rinny to teach you magic. The skills she teaches you in the order I've always got them is Drain Kiss > Electric Shock > Cock Growth. If it's still not working, it might be connected to the TF fetishes, which I think is what toggles dick/tit growth, so make sure you've got the Cock TF one on and try again