There are a plethora of reasons why things may not be working for you. The most common are fairly simple though, you're trying to run before you can walk or you're trying the wrong stuff.
It's agreed relatively unanimously that in order to understand how to cast spells effectively you need to have practiced through "the basics" for this I'd recommend taking a look over a few of the other posts on here which explain them pretty well.
If you've done that and things still aren't working out then take a look at what it is you're trying. I assume it's spell work as that's what many beginners come here to try. Firstly is the spell possible? Magic is not an easy answer and if the spell promises any miraculous results, physical transformations, materialising things out of thin air, etc then it's not real I'm afraid. Just use common sense, for example if there was a spell to make someone instantly wealthy then there'd be an awful lot more witchy millionaires running about, if you could become a mermaid then the Olympic swimming contests would be very different events, you get the picture. Secondly is the spell real, this is different to possible as some spells have just had more thought put into them than others. I'd always recommend designing your own spells but if you're not comfortable with that then at least screen which ones you try, if you've learnt through the basics it's usually fairly easy to pick out which spells are the most plausible.
If you want any more specific information then I'm afraid you're going to have to share what it is you're trying to do and how you're trying to do it to get a more precise answer.