What Zebrah is true.
You also, should take a look at your faith in magick. You have to know that what you're doing works. It's like placing your belief in a god or goddess. There isn't much power behind your faith, unless it is true.
Often times you must harden your beliefs, become resolute in what you are doing. If you have doubts as to whether or not your spell is worthwhile, then you should mull those doubts over from a different angle and consider either backing down from it, or trying even harder.
Many times, even for the most experienced casters, spells will not work simply because the powers that be (whatever one believes these are) see the cause or motivation unfit.