It's not a matter of believing karma or not believing it. Even if one does not believe in karma, it does not affect the course of karma.
Karmic cause and effect transcend the concept of double=slit experiment and quantum physics, where the act of observing a model, the observer becomes part of the model and hence will actively change the outcome. Karmic cause and effect is liken an ostrich burying its head in the sand. It does not make the circumstance disappear and changes nothing. Karma will play out its own destiny, with or without one's belief in its existence.
So, what do we have then. Well, since we say karma is not subject of believing in it or not, then what is so perplexing about karma?
The perplexity of karma lies in its concept and the understanding of it. Some do not believe in karma because they do not grasp how it works its destiny, like for example, believing in good begets good and evil begets evil and when they see karma not working in this manner, it is easy to reject its existence.
When one truly understands karma and how it works, you automatically will believe in it. But more importantly is not to understand the wrong concept of karma, for you will be truly disappointed when it does not play out the way you had assumed it should have played out. It's only your own perception and karma does not give a fucking damn to what you think or assume it is.
- Anki