I think I've found one possible cause for the fitting issues of the centre caps.
Studying pictures online (of my cheap Chinese ones and similar) vs the genuine set I have just acquired, the four retaining lugs that are pushed out by the spring are subtly different. 
The OEM ones have a vertical "slab" that is 10mm wide and 2mm deep with a couple of "buttresses" at the bottom. And of course the two-way sloping wedge section at the top with the spring pressing on the back.
The copies have the same slab, with buttresses, but part way up thicken to maybe 3.5mm and that extra thickness continues in the "wedge".
This makes them overall maybe 3mm larger in diameter where the lugs have to penetrate the hole in the centre of the wheel and of course much less flexible. 
Enough to make it difficult or impossible, unless (as I have) you use smaller/weaker spring rings and bend each of the lugs in a bit. 
I'll try to attach some pictures to show the difference.
Taken from different angles and just "snipped" from t'internet but the difference is pretty clear. The OEM ones have slimmer, straighter lugs and the copies are just too bulky.
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2003 D2 FL S8. Irish Green Pearl/Beige. Solar sunroof, auto-dim mirrors, electric rear seat functions, ski hatch retrofit; extended leather. Aftermarket DVB-T, reversing camera and full XCarlink (Bluetooth etc.).
2016 Volvo V40 T5 Cross Country (4WD) with ALL the toys including adaptive cruise etc. etc. Osmium Grey with Blonde/Charcoal leather interior. Polestar performance "optimisation".  (A much rarer model than a D2 S8 by the way!) Oh, and a brand new engine at just under 30,000 miles on the factory one! 
Finally: gone, but not forgotten.....
1998 D2 PF S8. AgateGrey/Platinum. Every option (I think) except electric rear seats, tiptronic steering wheel, ski hatch, towbar & dimming door mirrors.
e.g. Cruise control, NavPlus/TV, Bose, GSM, Xenons, Solar roof, Parking sensors, Alcantara/leather everywhere of course.  (internal dimming mirror added later)
1998 (very early) Ford Focus 1.8 Zetec; ABS/TCS, Heated screen/mirrors, Aircon, Auto-dim mirror, Leather, Trip computer, Cruise control, OEM Ford SatNav with CD changer.
And before that a lot of Rover 800s, a few oddities, a lovely Triumph Dolomite 1850HL with Overdrive and way back in my schooldays an Austin Seven aka Mini 850!
Last edited by HPsauce; 18th April 2015 at 11:50 AM.
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