This can happen if the person receiving the treatment believes the negative views of those around them, and then the divine realm has the handcuffs on, and cannot do nearly as much as would otherwise be possible. The issue of not producing a miracle in front of disbelievers is less a concern because in most cases there are enough precedents in medicine that people do understand odd things can happen—diseases, even fatal ones, can suddenly go into remission for no apparent reason. So this does not constitute proof of the Divine, and therefore, fly in the face of an atheistic perspective and override the free will of that individual to live in ignorance. In their skepticism, they will simply dismiss the miracle as one of those inexplicable turnarounds that are not yet understood by science, but most likely will be one day, at least as their thinking reassures them is the explanation.
So there is not as much risk in healing work of countermanding nonbelievers, and therefore, the divine realm is mostly working with the patient as far as whether or not the free will and belief quotient are sufficient to the task of healing. When the person is unable to communicate, if the higher self is in favor of the healing, this will also be enough to override negativity of caregivers and loved ones, but the damage may already be done if the impact of their negativity has impinged on the person through their awareness prior to a decline, leaving them comatose, for example. This may have to be factored into free will and allowed to stand, and healing denied. So it is difficult to make categorical statements here as there are many nuances and individual circumstances, and gradations of influence from many directions impinging on a particular circumstance. In general, there is a great need for further enlightenment about the truth of the Divine and what is truly possible for people to experience with a simple outreach that is not opposed by mixed feelings and beliefs.