Also, be sure you're casting within your range. Don't try to cast too powerful or complex of a spell right away, because you'll either not have the energy to cast it, or you will cast it wrong and cause it to backfire.
First of spells don't differ in power, thats just a urban myth, the power of the spell is determined by the caster, second what kind of spells, cause it would be smart to remember that any spell that is said to brake the laws of nature is fake and magic is never instantaneous or esthetic, thats just in the movies, a spell cant make something appears aot of nothing with a flash of light, also I would read and study up on magic before I would even think of doing a spell if I where you
Learn magick not spells. Spellwork is a part of magick but not the foundation of it. When someone jumps into spellwork before learning magick, it's like getting in a car before learning how to drive. It could very well be disastrous. You're lucky they didn't work! Some people end up doing spells that work perfectly, and it causes problems they don't know how to fix. But most likely, you are trying spells like eye color change, shapeshift, fireballs, wings, become a vampire, etc. Am I close? Or perhaps you're attempting the "kinesis" craze that is a waste of time and energy. Even if, even IF you could make a cup of water jiggle, cause a flame to dance, or crumple a paper clip with your mind (which is highly improbable) what good is it? David Copperfield and his stage magic will be far more popular than you, I assure you. If you want to do magic tricks, learn stage magic, it has more consistent results and it's overall more entertaining. If my friend told me to sit and watch as he moved a drop of water with his mind, I would pat him on the head and tell him it would be easier to just use a spoon with his hand.
Real magick is about fine tuning oneself internally first and then being able to manifest one's desires externally--desires not beyond natural laws.
First off, you need to know exactly what it is you are doing. Spell work is like baking bread. If you don't know how to work the oven, understand why dough has to rise or why you use certain ingredients and in what order - you end up with horrible bread. Magick is the same way. You can't go into it without the knowledge behind how it all works and why. You need to study first.
Look around this site first then check out some others. wwww.witchcrafted.org and www.witchschool.com are other useful sites you might want to look into.
Once you have the basics down, you will be able to cast a successful spell. Of course, a successful spell will not be like in books, movies or games. Want to point that out because of all I know, you tried some spells that are beyond the realms of possibility. And sadly, magick has its limits.
