It is indeed a great trial for a Muslim to make du'aa', sincerely and properly, continually, for a long period of time, and yet see no sign of response. Here, a person's eemaan in Allaah سبحانه و تعالى is truly tested. And here is the opportunity that Shaytaan uses to try to sow his whispers, and plant his doubts in a believer's heart. In this way a person might start thinking evil thoughts about his Creator, or start doubting the wisdom of the Divine Decree. So whoever falls into this, should immediately seek Allaah's سبحانه و تعالى refuge from Shaytaan, and turn to Allaah سبحانه و تعالى sincerely, making yet another du'aa' that he be saved from Shaytaan's evil plots, and that he be granted patience to withstand the test that he is undergoing. He should also ponder over his own limited knowledge and intelligence, and realise the infinite Knowledge of Allaah سبحانه و تعالى, and the extreme wisdom behind any Divine act.
The response to this question lies in understanding that the outcome of anything is dependent on the performance of the efforts that are necessary to procure it. In other words, it has already been decreed, for example, that a seed will give fruit if planted, but this will not occur unless the farmer takes the appropriate efforts in irrigating the crop, maintaining it, protecting the seedling as it grows, and ensuring as much as he can that the factors are amenable for the plant to give fruit. So even though a person believes in the Divine Decree, he must at the same time strive all he can to ensure that the desired goal occurs. So du'aa' is the means that one uses to achieve the desired goal that one has, and this means in no way contradicts the destiny that has been written for that person.
Du'aa' can be categorised in a variety of ways, depending on the perspective that one is using. Some of the more useful categorisations are as follows: 1. With Respect to the Actuality of Du'aa'
From what has preceded, it is clear that du’aa’ can only be directed towards Allaah سبحانه و تعالى alone. "Say (O Muhammad صلى الله عليه وسلم): 'I invoke only my Lord (Allah Alone), and I associate none as partners along with Him.'"(Surah Jinn, 72:20)