Maybe it's often just wrong expressed, meaning by doing the teachings, one gives way that Saddha grows for those knowing the teaching, but do not have sample.
As with all merits, one should never regret merits done. If with right intention, out of compassion, teaching, rebuking, encouraging someone, yet the hoped effect (hope here is actually what makes one suffer by fail) does not come to be, even if even understood as a harmful act by the potential receiver, it's his kamma. So it's not wise to regret goodness, similar as "I should not have feed my child".
Sure one might try to inprove ones skills or, if such happens to one to get bitten by giving food, better putting it indirect or let it be, if seeing that it bears no fruits.
The Buddhas way of training was first to encourage with benefits by doing good, then by dangers of doing bad, then both, and if all wouldn't work he would end to encourage.
“In using gentleness, [I teach:] 'Such is good bodily conduct. Such is the result of good bodily conduct. Such is good verbal conduct. Such is the result of good verbal conduct. Such is good mental conduct. Such is the result of good mental conduct. Such are the devas. Such are human beings.'
“In using harshness, [I teach:] 'Such is bodily misconduct. Such is the result of bodily misconduct. Such is verbal misconduct. Such is the result of verbal misconduct. Such is mental misconduct. Such is the result of mental misconduct. Such is hell. Such is the animal womb. Such the realm of the hungry shades.'
“In using gentleness & harshness, [I teach:] 'Such is good bodily conduct. Such is the result of good bodily conduct. Such is bodily misconduct. Such is the result of bodily misconduct. Such is good verbal conduct. Such is the result of good verbal conduct. Such is verbal misconduct. Such is the result of verbal misconduct. Such is good mental conduct. Such is the result of good mental conduct. Such is mental misconduct. Such is the result of mental misconduct. Such are the devas. Such are human beings. Such is hell. Such is the animal womb. Such the realm of the hungry shades.'”
But even if one could not receive Dhamma gifts, it's still valide to teach the Dhamma, sometimes it's even just thought for others, sitting by:
Gilana Sutta: Sick People .
In a relation, as long as a relation, both actually would have the duty to guide toward independency. Of course, if interests are not the same, a relation wouldn't work or go against ones interest, or is just used in cheating ways, just for the sake of relation or to gain certain things.
A real friend, how ever, does not fear to lose relations for the benefit of another, but speaks always what is benefical for the path toward aim:
What and why to speak? Here are, maybe useful, guidelines in regard of teaching:
Teaching the Dhamma At least, one is wise to stick with those who rebuke one in regard of what is proper, what not, for ones long lasting welfare
The Story of Thera Radha
While residing at the Jetavana monastery, the Buddha uttered Verse (76) of this book, with reference to Thera Radha, who was at one time a poor old Brahmin.
Radha was a poor brahmin who stayed in the monastery doing small services for the bhikkhus. For his services he was provided with food and clothing and other needs, but was not encouraged to join the Order, although he had a strong desire to become a bhikkhu.
One day, early in the morning, when the Buddha surveyed the world with his supernormal power, he saw the poor old brahmin in his vision and knew that he was due for arahatship. So the Buddha went to the old man, and learned from him that the bhikkhus of the monastery did not want him to join the Order. The Buddha therefore called all the bhikkhus to him and asked them, “Is there any bhikkhu here who recollects any good turn done to him by this old man?” To this question, the Venerable Sariputta replied, “Venerable Sir, I do recollect an instance when this old man offered me a spoonful of rice.” “If that be so,” the Buddha said, “shouldn't you help your benefactor get liberated from the ills of life?” Then the Venerable Sariputta agreed to make the old man a bhikkhu and he was duly admitted to the Order. The Venerable Sariputta guided the old bhikkhu and the old bhikkhu strictly followed his guidance. Within a few days, the old bhikkhu attained arahatship.
When the Buddha next came to see the bhikkhus, they reported to him how strictly the old bhikkhu followed the guidance of the Venerable Sariputta. To them, the Buddha replied that a bhikkhu should be amenable to guidance like Radha, and should not resent when rebuked for any fault or failing.
Then the Buddha spoke in verse as follows:
Verse 76: One should follow a man of wisdom who rebukes one for one's faults, as one would follow a guide to some buried treasure. To one who follows such a wise man, it will be an advantage and not a disadvantage.
By having given into the Sangha, as a side message, even if normally not a desireable person for it, one can expect to be never cast off anyway, even it currently mental or physical blocked.
This is why it is good to encourage even those not so much faith to give toward the Sangha and very clear taught to any child of good family.
And as a matter of gratitude, again, one who was given the way to meet the Gems, received Dhammaguna, such one is always obligated to help that a Brahma does not fall from a good path or to direct him there. Like a child could pay the goodness of parents, only by ways of guiding upward and beyond, back.
So maybe good to just remember that one possible had bitten others when they had encouraged one to better, to understand possible being touched if reject.