it really depends on the spell and the caster. the spell might not be a working spell [say any spell that would go against human nature, flying, transformation etc.] or it calls for the wrong ingredients [say green candles for a love spell] then these spells won't work. the other way would be if the caster doesn't charge enough energy and focus well. you can't just light a candle, read some words on a page, then watch tv as the candle burns down. you need to clear your mind, feel the energy flowing, visualize your intent, then work to achieve the goal.
while not every spell on the site works, try looking at it's ranking [and no, not all high ranked spells work either] you also need to think about it logically, ask yourself 'does this spell make sense? will it work?' research the ingredients, google is your friend, just type in 'magical properties of (ingredient)' and you should get your answer. as you continue on, you'll be able to figure out working and non-working spells easier. for now, just gather as much information as you can.