Karma is not a universal justice system; it means that your actions will have a consequence, which will have a consequence, which will have a consequence, etc. Karma should be viewed more as a net then a cycle because consequences spread in the same way.
If you’re mean to someone you may put them in a bad mood, in that mood they will probably be of bad temper and be equally mean to the people around them and put those people into bad moods and so it goes on. Smile and the world smiles with you
But that isn’t really the point of the question so I will say, yes I believe in karma, but it doesn’t always have effect.
Sorry, I seemed to have gone on a little bit of a ramble about karma!