Welcome to SoM Katie! I hope you have a good time here.
There are many reasons that a spell could not work or go wrong in any way... way too many reason to count. Try going through your previous procedure and think about what could be approved upon - should you use more herbs, or less herbs, maybe a different kind/color of candle, those sorts of things. There are no wrong answers, go with your gut feelings on what you can do. Intuition is a big part of spell crafting and casting.
Making a spell is a detailed process that takes practice, but of course, you have to practice first to make perfect. Here are some good links below on spell crafting, enjoy!:
Read those links through and through and be creative!!!
If you have further questions, feel free to mail me, write on the forums or google search your question. Remember, as with any endeavor about information you are seeking, google is your best friend!