Educate your teeth!
Patients frequently ask why a crown keeps falling off. A properly done crown will not fall off unless part of the tooth, filling, or post supporting the crown fractures away.
If nothing under the crown or bridge is fracturing away when the crown falls off, then the problem is invariably related to poor design. A crown or bridge depends on the shape of the tooth under it to stay on. This special shape is created by your dentist when he prepares the tooth. The tooth must have adequate length opposing walls which are very nearly parallel to one another for the crown to stay on. If the walls taper toward one another too much or are too short, the cement will not hold and the crown or bridge will fall off.
Additionally, improper design can include incorrect contact with the opposing teeth. Repeated bumping together of teeth in an abnormal way can result in your crown or bridge being dislodged.