Supposedly, something about the neural pathways was stopping them from fully "cloning" or "transferring" consciousness.
In other words, they were having a hard time recreating the neural pathways in the new host, and it turns out, the neural pathways are what make us…well.. us. Conscious beings with memories and experiences. So they could clone someone, no problem. But getting them to be 'themselves' was where they stopped. The new clones would essentially start out as blank slates. Big problem if your goal was to live forever.
>Sounds a lot like Westworld….hmmmm….