There are a handful of reasons as to why your spells may not be working. It may be any one or more of the reasons below,
1. You have not yet learnt the basics and do not fully understand how to cast the spell successfully.
2. You do not believe that the spell will work and thus do not put in the required energy.
3. The spell is fictitious and will not work. Physical transformation spells, anything to control the elements and generally anything that is also impossible without the use of magic will not work.
There may be a few other reasons but they are by far the most frequent causes as to why spells do not work.