There are many things that can cause a spell for good luck & fortune to go wrong. Your doubts, your emotions, negative thoughts, or even negative energies in the air. The list goes on! It could even be your lifestyle or there is a lack of action in obtaining the good luck. You attract the same vibrations that you give out.
For example: You cast a spell to get a good grade on a test, but you did not study.
Those are two different vibrational frequencies (or energies). So, I recommend that you envision yourself with the luck you need. This is a key part of manifesting something into your life and how people get to the point of success in their lives. It is almost like a "plan" or some kind of vision board. You might need to cleanse the space around you and get rid of any unwanted energies. The thoughts & energies from other people can have a great effect on your spell work. The reasons what could of happened are endless.
Why do you think it failed? Is something happening in your life that may result in the bad luck? Are you using a good luck spell to get rid of bad luck?