This is a known issue. There was a part change on the cap after '10 I believe. One key thing is to keep the cap rubber ring clean as well as the mating surface. Make sure neither surface is dry, or the rubber will bunch up. You can also gently bend the fasteners to make a tighter seal.
08 E90M3 6MT Alpinweiss/Schwartz/BPMSport 91 oct tune/BPMSport feature coding/BMW Pedals/Ultimate Clutch Pedal/Dinan Exhaust/BMW Alcantara wheel/BMWCCA #453468/Rear Fog Switch