Well I've had plastic caps on, and i keep having to replace them, either i have too much pressure in my tires (which i doubt is the case cause they always look flat), or I'm hitting bumps and stuff and they pop off, I've heard of that. But I'm not really sure what the problem is, at first it was just one tire, then i aired up a tire today and noticed all 4 were off. And i didn't screw the plastic ones super tight, just to where they were on all the way. Im not having a problem with my lugs at all, they were and are torqued correctly.