I just picked up a set of stock center caps for my stock steel wheels. When I tighten the plastic nuts down onto the lug nuts, the center cap is still loose. Are there different lug nuts that I need or can I replace the plastic nuts on the center cap? I'm dissapointed that they're not working.

 

The threads in the nut caps may be stripped out. They can usually be replaced but are a little expensive considering what they are.

 

Just to update, the caps were definitely too long. I went to autozone and found some that I believe are for older trucks. They're shorter. They are too damn expensive but I just got enough for 2 for each wheel for now and kept the long ones on everywhere else for looks.