Allah (SWT) has promised in Quran to respond to the supplications (Your Lord said: Supplicate to me, I will respond to you). (Sura 40, Verse 60).
No supplication can go with out respond as far as it is for a permissible cause. The point is : When is the time that we see the response of our supplications?
We want to see it soon because we don't know the future and we think that it is good for us now, but Allah (SWT) knows everything including the future and the results which will happen if we get what we supplicated for. He knows that some wishes can be harmful under certain circumstances, that is why He does not grant it in the wrong time because He does not want to harm us. Some of our wishes can be delayed to be fulfilled for long time till Allah (SWT) knows the time when it will not be harmful for us to get it. Some wishes can be harmful for us all our life, so, Allah (SWT) grants us His response in the hereafter with great reward and ample bounties. Hadeeth says that when the believer sees the response of his supplications in Paradise, he will wish all his supplications should have been postponed till hereafter.
Believer should continue supplicating all the time and under every circumstance, with full confidence that Allah (SWT) will definitely respond and grant him the best in the suitable time which is known to Allah (SWT).