Depending on the spell you cast, the actual spell might be the reason that it didn't work. For example, if it included changing shape, turning into a creature, growing wings, fur, horns, tails, extra limbs etc., changing eye color, gender, race, age... It won't work because these things are impossible. But, if they were reasonable, like attracting wealth or protection, they most likely didn't work because A: you didn't believe it would work, B: your energy wasn't really directed at what you wanted to accomplish.
A tip everyone should follow is to learn proper grounding techniques. Pulling energy from the Earth or universe can be tricky st first, but very valuable. Connect your energy and the source of your desire to manifest something. Visualization is very good for this type of thing :)
Imagining roots connecting you to the core of he Earth is what I do. Good luck :)