Two ideas on the cap: (1) The vent could be plugged (duhh). Is the bike new to you? Or is the cap newly replaced? Does the lock work, or has it been "fixed" by epoxying the lock cylinder in place either by you or a PO? (My ST had the epoxy job, but the PO did punch vent holes).
(2) Could the cap have been replaced with one of special ones that go on the special emissions equipped tank systems? (Can't remember the acronym for that system, but have seen them in the Max BMW fiche).
Either situation could be remedied with judicious application of a 1/16 or 3/32 drill bit in the appropriate spots to open plugged vent holes or create new ones.
I'm saving my good BMW locking cap on a shelf and running a Stant locking cap for late model Ford/Mercury cars. Becasue its from the vapor-tight emission era of today, I had to apply said drill bit up through the bottom and put about 3 holes up through the inner core which hold whatever kind of check valve paraphanalia lives in there. Works great.
Some anecdotal info only slightly related to the topic for those interested in using the Stant automotive caps - The threaded portion of the car caps like on my Town Car gas cap are identical to the BMW caps. The gaskets at the top of the threaded portions are identical. For bikes that have the outer trim ring seated in a groove on the BMW caps like my ST, 1/4" wide self adhesive weatherstripping wrapped around the outside of the cap with enough wraps to make a friction fit when slipping the trim rim over the bottom will hold it in place just fine. And if you wrap it just below the top of the cap the trim ring sits correctly and the weatherstrip is unnoticeable.
And a side note: While I was playing with the car's OEM gas cap and sorting all this out I discovered that the Lincoln gas cap turns in its little plastic tether which is fitted into a groove identical to the groove on a BMW cap which holds it trim ring. I came really close to going and buying a Genuine Motorcraft cap from a dealer, fitting the BMW trim ring, and seeing how many people would ever notice the logo.
But I settled for the Stant so I could have a locking cap.