Topic: Wheel cover walking

[truetriplex]

I have a full wheel covers that "walks" around the rim.  I kept pulling it off and re-centering it to the valve stem, but the problem persist.  As the wheel cover moved on the rim, it finally cut the valve stem and the rest is history (along with the tire).  First question is what causes this? and second, how do you fix it?  I have the same problem with the other three rims, but not as bad.