A crown that doesn't fit correctly over your tooth can lead to tooth decay. If the cement that attaches the crown to your tooth washes away, bacteria invade the space between the crown and your tooth. Crowns can also become damaged over time. When a crown is damaged, it becomes easier for bacteria to enter into the tooth underneath. The bacteria excrete acids, and the acid stays there for a prolonged period of time since there is no saliva.
When a person fails to take good care of their mouth, their gums are susceptible to infection, and bacteria can make their way to the tooth that way.