Perhaps I have just chosen my ferals carefully ... I have never failed. Once the trust is established [it usually takes me about six months] they become overly trusting and I have had to keep them indoors.
A word of warning: Do not bring a feral cat into your home with your established, healthy cats. I have the means to keep a feral in a completely separate building while they are being tamed. They must be vetted immediately and then, after that, kept separate for quite a while to be absolutely certain that they do not bring anything harmful to your other cats.
Anyway, I think because that kitty was so tiny and she was trying to fend for herself without her mom, she just never mentally recovered. At five years old, she was still terrified of everything, and we could never get her litter box trained. I think with her she probably suffered from some sort of developmental damage due to the terrible circumstances when she was a baby. Not only that, but you know how other animals will try to weed out the sick and weak? Well, this poor kitty was always being treated badly by my many otherwise very loving kitties. They knew something was wrong with her. She had to go where she was an only kitty.
The other was a starving kitty I found at a rest stop on an interstate. Beautiful, and very sweet. But she was extremely aggressive. She would search out my other kitties just to beat the crap out of them. We had her completely vetted, healthy, and spayed, then found her a new home, too.
I have also rescued two others (one my mom has, the other one of my sisters has), that have never seemed to recover from their lack of human contact. They will let their owners pet them, but frantically hide from any other human contact.
I guess I have just had so many, their personalities run the gamut.