The "claws" on the plastic variety of caps need some help to stay put over
time. Just like dentures! Sorry for the graphic analogy. A small amount of
Si Seal, Goop, Moisture resistant glue (need not be water proof.... unless
you are driving Roadhaus to Europe) on the outside tip of the "claw" will
usually secure the Cap under most conditions. I would think you are not
looking for a perminate solution... meaning you still want to be able to
remove the caps to inspect the front bearing grease-caps or rear axle-nuts
etc? A bit of Si Seal ought to do it.