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.
2002 E39 M5 / TiAg / Tubi Rumore / E60 SSK / Blanton 3.45:1 7 disc 40/100% locking diff / NCS Coding / Colby Alcantara / StrongStrut STB