The core is to be born in a specific time period; they have to choose from the available Patterns of the pre-born KA (which are later referred to as ego-personalities). This choice is approximate, since they are able to select vaguely; the reason for that is that the ego-personality of someone develops in the way that is discussed by psychology. The person's environment, as well as own choices shape their path, and their own personality. So the KA (personality) is under development for a large part, if not for all, of a person's life.
The ontological core (BA) therefore may choose wrongly, but they try to select the best candidate, according to their own judgment, from those available, presented before them as pre-born Patterns. If the person develops to someone who opposes their core by their lifestyle, actions etc, then the core remains dormant and does not manifest in their life, waiting for another chance, after this life ends. The core (BA) does not dominate the ego-personality (KA); they respect the free will of the person, otherwise all people would be forced to do certain spiritual things (which they don't). Freedom is ensured, even though it's not time and cost-effective.