The absolute biggest obstacle to a spell's success is over-thinking the spell. You have to trust in the Universe to manifest your wishes. Obsessing and over-thinking work to counter-act that trust.
Think of a cast spell as creating a giant "rock" of energy that is rolling down a hill toward manifesting your goal. Every time you question its efficacy, every time you worry and fret, you are in effect chipping away at this rock of energy. Some times all you have left is a little pebble.