If these have the little "teeth" on the inside that digs into the paint around the rim, or snaps over nubs on the wheel, the solution is as close as a needle nose pliers away.
Just grab those teeth and bend them out a bit, so that they bite into the wheel harder.
I lost a '54 Buick hubcap a couple years ago, and noticed that I had to keep an eye on the others, and tap them back on snug about once a week. After an hour with the pliers, they went into place, and never wanted to leave again.
-Brad
