Why does it seem that some prophecies are not fulfilled?

The Promised Messiah ^as wrote:

Unfortunately ignorant people do not understand the difference between the promise (وعده) of God and His warning (وعيد). In reality, there is no promise in a warning. It is only that God, because of His Holiness, requires that a wrongdoer be punished, and sometimes He informs recipients of revelation of this as well. Then, when the wrongdoer fulfills that requirement with repentance, seeking forgiveness, and humility, then the requirement of the mercy of God exceeds the requirement of displeasure and it hides that displeasure within itself.

(Ruhani Khaza’in, vol. 15, p. 537)

The Promised Messiah ^as wrote:

It is an accepted principle of the whole world, and an agreed upon doctrine of the people of Islam, Christianity, and Judaism, that a warning, as in a prophecy of punishment, can be withdrawn even if it did not specify the condition of being withdrawn through repentance, seeking forgiveness, and fear.

(Ruhani Khaza’in, vol. 15, p. 535)

Every prophecy of punishment carries the condition of this principle of the Holy Quran:

And Allah would not punish them while they sought forgiveness.

(Surah al-Anfal, 8:34).

When the people of Nineveh repented by accepting Hadrat Jonah ^as and saved themselves from the prophecy of punishment, they did not prove him ^as false. Rather, their repentance was a testament to his truth. When a prophet of God prophesies a punishment, that prophecy is fulfilled either with the people accepting the warning and repenting, or with the people rejecting the warning and being punished.