How can jinn harm those who regularly recite the prescribed remembrances (adhkar) and are under God's protection, such as a jinn reaching girls and committing fornication with them, even though the verses and hadiths of protection state that reciting them safeguards a Muslim from all evil? And what is the explanation for that?

The words of the Prophet are truth. The people of supplication differ in the answering of their prayers according to the strength of their faith. Ibn Al-Qayyim likened supplication to a sword, its effect being determined by the strength of the supplicant's faith, just as the impact of a sword's blow is determined by the strength of the striker's arm. The same applies to the Qur’an: the stronger the faith of the one being treated with it, or the one reciting it for healing, along with avoidance of sins, the greater its effect will be in healing, by Allah's permission. Ibn Al-Qayyim mentioned that the Qur’an is a complete cure for all ailments of the heart, body, and those pertaining to this world and the Hereafter. If the sick person treats himself with it sincerely, with faith, complete acceptance, firm conviction, and by fulfilling its conditions, no disease will ever resist it. There is no disease except that the Qur'an contains an indication of its cure, its cause, and the means to protect oneself from it. This is in accordance with the Almighty's saying: "Is it not sufficient for them that We have sent down to you the Book which is recited to them? Indeed in that is a mercy and a reminder for a people who believe." So, whoever is not cured by the Qur’an, may Allah not cure him, and whoever is not sufficed by the Qur’an, may Allah not suffice him.