Why does there sometimes seemed to be no impact from shar‘i ruqyah?
Question

Shar‘i ruqyah is permissible if a non-Muslim does it for a Muslim, and he may be healed by it. There is someone who did ruqyah for poison that reaches the blood, without pumping the stomach, or any change, or any blood transfusion, and he was healed completely. The same happens in cases of witchcraft, jinn possession, epilepsy and the evil eye – these problems may be resolved completely through ruqyah. But a Muslim may do ruqyah and the sickness continues for a long time, or the person thinks that he will never recover. What is the measure by which it may be determined that the ruqyah has started to work straight away or is delayed, and what kind of sicknesses can ruqyah be used to treat them? How can we determine the quality of ruqyah?
Praise be to Allah.Firstly
Ruqyah by reciting adhkaar and Qur’anic verses is Sunnah
Ruqyah is a type of supplication (du‘aa’); hence it is prescribed to do it by reciting verses of Qur’an and adhkaar that are prescribed in the religious texts.
An-Nawawi (may Allah have mercy on him) said:
As for ruqyah using verses of the Qur’an and well-known adhkaar, that is not prohibited; rather it is Sunnah…
The scholars narrated that there was consensus on the permissibility of ruqyah using Qur’anic verses and words of remembrance of Allah, may He be exalted (i.e., adhkaar). Al-Maaziri said: All ruqyah is permissible if it is done by reciting verses from the Book of Allah or words of dhikr. It is prohibited if it is done in foreign languages (i.e., not Arabic) or words of which the meaning is not known, because of the possibility that they may be words of disbelief (kufr)." (Sharh Saheeh Muslim 14/169).
Supplication (du‘aa’) is one of the means that Allah, may He be exalted, has ordained for healing.
Is the healing outcome of ruqyah guaranteed?
Taking appropriate measures does not guarantee results, as is well known, so the result may not be achieved because of not meeting all the conditions connected to the measures, such as if one who is heedless recites ruqyah; or because of some impediment, such as if the one for whom ruqyah is done has opposing ideas and does not believe that it is beneficial.
Ibn al-Qayyim (may Allah have mercy on him) said:
The Qur’an is complete healing from all sickness, both spiritual and physical, whether in this world or the hereafter, but not everyone is qualified to benefit from that or will be able to seek healing thereby. If the sick person seeks healing with it in an appropriate manner, and seeks a remedy for his sickness with sincerity, strong faith, complete acceptance and certain belief, fulfilling all the necessary conditions, no sickness can ever resist its impact." (Zaad al-Ma‘aad  4/322).
But in His wisdom, Allah may delay healing or prevent it.
Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) said:
If a person offers supplication and does not overstep the mark therein, he will attain what he seeks or something similar. This is how the response is. For the specific thing that is sought may not be possible, or it may cause harm to the supplicant or someone else, but the supplicant is unaware of that and does not know what harm it may do him. The Lord is ever near and responsive, and He is more merciful to His slaves than a mother to her child, and He is the Most Generous, the Most Merciful. When He is asked for a specific thing and He knows that it will not do the person any good to grant him what he has asked for, He will give him something else that is equivalent to it, as a father does for his child if the child asks for something that he does not have; he will give him something equivalent to that – and for Allah is the highest description." (Majmoo‘ al-Fataawa  14/368).
Ibn al-Jawzi (may Allah have mercy on him) said:
It is a sign of ignorance for a man not to be aware of what is meant by his being accountable, for accountability is designed to be the opposite of what he wants and desires. So the man of reason should feel at ease when he gets the opposite of what he desires. When he offers supplication and asks to attain what he desires, he should realise that he is doing an act of worship by offering supplication; if he is given what he wanted, he should be grateful, and if he is not given what he wanted, he should not persist in asking, because this world is not for getting what one desires; rather he should say to himself (interpretation of the meaning): {and perhaps you love a thing and it is bad for you} [al-Baqarah 2:216].
One of the worst types of ignorance is for a person to feel disappointed in his heart for not getting what he desires; perhaps he may object in his mind, or he may say: Getting what I desire does not do any harm, yet my supplication is not answered!
This is indicative of his ignorance, lack of faith and failure to surrender to divine wisdom." (Sayd al-Khaatir  p. 625-626).
Allah, may He be glorified and exalted, may respond to the ruqyah of a disbeliever and bring about healing through it so as to establish proof against him (the disbeliever). Hence Allah answers the supplication of the disbelievers at times of calamity, to establish further proof against them.
Allah, may He be exalted, says describing them (interpretation of the meaning):
{And whatever you have of favor - it is from Allah . Then when adversity touches you, to Him you cry for help.
Then when He removes the adversity from you, at once a party of you associates others with their Lord.
So they will deny what We have given them. Then enjoy yourselves, for you are going to know} [an-Nahl 16:53-55].
In His wisdom, Allah, may He be exalted, may delay healing for a believer, for a purpose such as such as increasing his reward, rewarding him for his patience, and so on.
Conclusion: the outcome of ruqyah is subject to the will of Allah, may He be exalted, and He is the Most Merciful, the Most Wise, the All-Knowing.
Secondly:
As for the guideline on the quality of ruqyah, it is like the guideline on du‘aa’ and dhikr; there is no stipulation except that the one who does it should be a Muslim, honest and sincere, and he should use the ruqyahs that are prescribed in the religious texts.
For more information, please see the following questions: 7874 , 13506 and 36902 .
And Allah knows best.