Allah (SWT) has promised in Quran to respond to the Du'a (supplication) and there is no Du'a with out response. (Your Lord said: Seek from me, I will respond to you) وقال ربكم ادعوني أستجب لكم(Sura 40, verse 60).
We do not know how and when the respond will come. Allah (SWT) knows the best time and best way to respond to any Du'a. Our expectations are according to what we know, but Allah (SWT) knows everything and He is The Most Merciful, The Absolute Wise, so we should leave it to Him. Our mercy on the oppressed can never be more than Allah's mercy on them. He will definitely compensate them in the best way in the best time.