Of Allah’s many Beautiful Name’s is Al-Hakim, the Most Wise. To Him belongs the greatest wisdom; all of His Decrees are Wise. When He gives, He gives with wisdom; and when He prevents, He prevents with wisdom. As Allah says:
“Perfect (or Complete and All-Encompassing) Wisdom.” (Surah al-Qamar:5)
So it is possible, that what we desire is not wise for us have. And because of this, it is not granted to us, or it is delayed until such a time that it is suitable. It is possible that the du’a might be responded to at a later time, due to factors that Allah alone is aware of. For example, a couple might pray for a child, and their du’a is not answered for a number of years, since Allah knows that it would be better for them to be blessed with a child at a later date.
So the true believer, accepts Allah’s Divine Wisdom, and does not let a delay in response swerve them from the true imaan in Allah.