I have thought about this question and have also been asked this question.
Before learning Buddhism and Lamrim, I would have been stumped for a coherent answer.
With understanding the four aspects of Karma (check out Master Ri-Chang’s Lamrim commentary here, Track 50B: , it all seems so much more obvious:
1 The certainty of karma:
业决定理者
2 The magnification of karma
业增长广大者
3 Not experiencing the effects of actions that you did not do.
所未造业不会遇者
4 The actions you have done do not perish.
已造之业不失坏者
All these 4 aspects will essentially confirm that everyone experiences Karma - your bad or good deeds (the actions) will catch up on you one day as suffering or happiness (effects). 
Here is another link to a discourse by Venerable Thubten Chodron on the General Characteristics of Karma where one answer has been provided:
When we’re thinking and meditating about karma in this way, it’s very helpful to make many examples in our life. The question often gets asked, “Why sometimes do good people have unhappiness, and people who are harmful have good results?” Well, there are certain conditioning factors in this life—social systems and stuff like that. But there are also karmic things. A person who does a lot of harmful actions this life but experiences some degree of fame or wealth is consuming their good karma that they created in previous lives. They’re consuming it by having fame and wealth, but they’re also creating a ton of negative karma that’s going to lead them to unhappiness in the future.
Sometimes we see very wonderful people experiencing suffering in this life. Some of that suffering may be due to diet and external conditions, social systems, and so forth. But some of it may also be due to negative actions they did in a previous life. This way of understanding can be very helpful.
I do not recommend explaining this to people when they are in the middle of grief when they have no understanding about karma. This is not a skillful way to introduce karma to people who are grieving and who don’t have faith in cause and effect. I say this because they very easily misinterpret it to mean we’re blaming the victim and saying they deserved to suffer. We‘re not blaming the victim and saying somebody deserves to suffer. We’re just saying causes bring results and results happen because of causes. Nobody deserves to suffer, nobody is worthy of suffering. As much as possible we should do what we can to alleviate suffering.
hope this helps in our practice. happy contemplating! 
