The first and most common reason I have discovered is that you are already in possession of the answer. It was given to you by God and you have it, but you don't recognize it. It would be better to stop asking for an answer and instead ask to be able to see what you have already been given.
The second reason is that you need to struggle and make your own decision first, then to petition to know if the decision is right. It is not always appropriate to defer all decisions to the Lord. You must develop the capacity to make sound decisions on your own. The Lord will, of course, ratify the correct decision and warn you about the wrong one. But you need to develop the ability to decide first. (D&C 9: 7-9.)