There are many reasons why a spell on this site might not work.
First of all it might be something which magic cannot do. Magic does have its limits. Spells will not let you shape-shift, turn into a mythical creature such as a vampire or werewolf, change your physical shape or the color of your eyes, fly, practice any form of kinesis, etc. None of the spells in the Fantasy section here, for example, will really work.
Secondly, the spell might not work because you have not studied the mechanics of magical workings nor mastered the necessary skills to make a spell work. It takes more than simply finding a spell on a website, buring a few candles and chanting some words. If you don't first understand how and why magic works then spells aren't going to work for you, at least not consistently.
And thirdly, no spell works 100% of the time. If you're tired, sick, angry, or simply not in the proper frame of mind it is unlikely that a spell will work. If you aren't properly focused, have not visualized the outcome of the spell adequately a spell will probably fail. And sometimes the Universe just says no, and a spell won't work.
Yes, there are a number of spells on here that are fake. But there are many spells on here that are real and that can work if you know what you are doing.