To get the desired results, all conditions must be fulfilled and all impediments removed.

Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) said:

Just taking appropriate means and measures does not necessarily lead to the expected results. If rain falls and seeds sprout, that is not sufficient for vegetation to grow; rather it is also essential that there be fecundating winds, by Allah’s leave, and it is essential that there be an absence of impediments.

It is essential for all conditions to be met and all impediments to be absent, and all of that happens by the will and decree of Allah.

A child is not born as the result of merely depositing semen in the vagina. How often that happens, but no child is born.

Indeed it is essential that Allah should will to create the child, then the woman conceives, then the embryo begins to grow in the uterus, and all other conditions that are needed, and the impediments that must be absent, for the creation of a child.

End quote. Majmoo‘ al-Fataawa (8/70)