وَلَقَدْ وَصَّلْنَا لَهُمُ الْقَوْلَ لَعَلَّهُمْ يَتَذَكَّرُونَ (And We have conveyed (Our) words to them one after the other, so that they may take to advice. - Al-Qasas 28:51 ⧉). The word وَصَّلْنَا (wassalna) is derived from وتَصِیل (Tausil), which literally means to strengthen the rope by adding more strings to its strands. Here it means that Allah Ta` ala has maintained the continuity of guidance. Certain advisory subjects are repeated in the Qur'an in order to make them more effective.
Certain rules for preaching
It shows that an important trait of preaching carried out by the prophets was that they used to convey the truth to people continuously. Rejection and falsifying of truth did not deter them at all from their mission. Instead, if someone did not listen to them the first time, they repeated it the second time, and if they did not succeed even the second time, they used to reiterate it a third time, and so on, without showing any sign of exhaustion. It is true that no preacher or sympathizer has power to change one's heart, but what the prophets could do was to keep on making their efforts without being dishearted or exhausted. Even today the same principle applies, and those who preach should take a serious note of it.