Just trying to understand karma a bit better

So they say whatever happens to you is your own karma that comes back. What you get is what you probably gave someone in some past life, but what about what you don't get? Ex: we often see cases where someone was preparing for some career and ends up doing something else. Students who don't end up getting a seat even after studying so hard for years but someone who studied for two months gets a seat. If hard work and sincerity do not qualify as good karma in such a case then what does? How would past life karma come in the picture in such scenarios where you put in sincere effort and yet you don't get favorable results.

This is just an example..... everyone goes through such experiences in life where they don't end up getting what they've worked for. Would like to hear other's opinions and thoughts on this