It's all that f*cking wax!
I had exactly the same problem when I had a new BINI Cooper a few years back - after only a couple of weeks, the rear plate just slid off, as some muppet hadn't removed the original protective wax coat prior to sticking the plate on.
Velcro FTW, I say.
There's something about those rims - at first sight they look minging, but then they do something and just seem to work after a while....