Why do spells work and sometimes they don't work? There are many things that contribute to a spell or ritual work properly and not one thing alone. All of the following combined together is what will make a Spell, Hex or Curse work and work good.
The proper Moon phase
The proper Planetary Hour and Day
The proper materials and supplies
The mental condition of the Practitioner
The will of the Universe
The experience of the practitioner
The Magical Energy built up by the Practitioner
Your belief in the magic you are working
The proper Spell
Read about those things and your spells will work better, there are not actual spells that are made for novice people, but try choosing simple ones and use the criteria i told you now.