There is an ancient tradition that the Mezuzah protects the home in which it is placed and the people who live in that home. It is believed that the Mezuzah even protects the inhabitants when they are outside of the home. This protection only holds true if the Mezuzah is a kosher one. If for some reason the Mezuzah becomes Pasul (not Kosher), the protection is no longer there.
At our store in Los Angeles, Mezuzah Gallery, we provide Mezuzah and Tefillin checking services. I am the one who checks all of the Mezuzahs and Tefillin. You are supposed to check your Mezuzahs at least every 3.5 years. Many Rabbis recommend having them checked every year. Often people will also have their Mezuzahs checked if someone gets ill or injured (H.Y.).
Occasionally when this happens, we see a very compelling connection between the problem that the person was having and the problem in the Mezuzah. For example, you may have someone who has an eye problem and when we examine their Mezuzah, we find that there is a crack in one of the letters of the word "Einecha" (your eyes) in the Mezuzah.
All of this being said, I must point out that having a non-Kosher Mezuzah doesn't cause bad things to happen. It is just that without Kosher Mezuzahs on our door, we are lacking the special divine protection that G-d offers to those who fulfill the Mitzvah of Mezuzah properly. From time to time I will post some of the stories that happen in our store. I will of course leave out the name or change it to an alias to protect the privacy of our customers. Keep your eyes open for the first story in this series which should be coming soon.