Problem with hub caps

[1-PU70]

Here is a pic of the truck with the full hup caps. The wheels are 15X8 ralley wheels, the ones that use beauty rings and center caps.The problem is the hubcaps move on the wheels. When I first put them on the valve stem is in the center of the notch for them. After a drive the valve stem is off to one side of the notch and pushed over to the point the valve stems are geting cut by the hub cap. What can be done to keep the hub caps from spining on the wheels?