Karma is a natural law like any other natural law

Sometimes we face certain situations where, although we have done something good for others, we may not be able to reap the consequences within this lifetime. When we are talking about the law of causality, we are not limiting its operation to the confines of this life alone, but rather are taking into account both this lifetime and the future. Occasionally people who do not have a proper knowledge of karmic law say that such and such a person is very kind and religious and so forth, but he always has problems, whereas so and so is very deceptive and negative, frequently indulging in negative actions, but always seems very successful. Such people may think that there is no karmic law at all. There are others who go to the other extreme and become superstitious, thinking that when someone experiences illness, it is all due to harmful spirits.... It is also possible for very negative people to experience their positive karma ripening immaturely due to the very strong force of negative actions, and thus to exhaust the potentials of their virtuous actions. They experience a relative success in this life, while others who are very serious practitioners, as a result of the force of their practices, bring upon this lifetime the consequences of karmic actions which might have otherwise thrown them into rebirth in lower realms of existence in the future. As a result, they experience more problems and illnesses in this life.

Just resolving not to indulge in a negative action is not enough. It should be accompanied by the understanding that it is for your own benefit and sake that you must live with awareness of the law of karma: if you have accumulated the causes, you will have to face the consequences; if you desire a particular effect, you can work to produce its causes; and if you do not desire a certain consequence, you can avoid engaging in actions that will bring it about. You should reflect upon the law of causality as follows: that there is a definite relation between causes and effects; that actions not committed will never produce an effect; and that once committed, actions will never lose their potentiality simply through the passage of time. So, if you wish to enjoy desirable fruits, you should work for the accumulation of the appropriate causes, and if you want to avoid undesirable consequences, you should not accumulate their causes.... [Karma] is a natural law like any other natural law.

--from Path to Bliss: A Practical Guide to Stages of Meditation by H.H. the Dalai Lama, Tenzin Gyatso, translated by Geshe Thubten Jinpa, edited by Christine Cox, published by Snow Lion Publications