Learning requires 'safety' - plus just enough 'discomfort'
The critical success factor is the quality of the coaching relationship. Good coaching outcomes are co-created in the safe "transitional space" this relationship creates. Connection is what allows the coachee to explore deeply held beliefs and assumptions they never previously dared to question.
Both parties have to recognise and deal with the emotional "change curve" - have a look at this version ("the hero's journey") - because any significant change involves a dip in self-certainty, with corresponding feelings of discomfort that can cause people to give up or retreat from change.
Line managers engaged in (or sponsoring) coaching may also find it helpful to look at this version of a very famous "situational leadership" model ("sinking or swimming"). It clarifies how your style may need to change as people develop their competence and move along the change curve.
Finally, coaches (and coachees) need to accept that coaching rarely goes straight from A to B (as sometimes implied by the GROW model). The reality is more a 'random walk' within the boundaries of what you can visualise as a "development diamond" - a shape that represents opening up to explore, then closing down to action. That may require more than one conversation!