While i can't compete with, Lark. You can never actually know a spell, will always, work. It,doesn't!
You have to know what you want from the spell, and, you have to believe it will work.
Even if you visualize, ground, meditate, and have your will cut out, it may not work. People say, try to use the ''mundane'' way. As in, ''Try to use the human way first, with all efforts you can give'' If it doesn't happen, then you can use magic. Sometimes, it may not work,for it defines nature. Well how, will it not abide by nature? Well,first off, a healing spell, i would say would work. But, if your trying to cure cancer?That won't work, it's seemingly impossible, to me!