FOr some people, the gas will sit in the tank, unused for a very long time. While the system is reasonably well sealed, I wonder if it's losing its more volatile components over time, or, if you're in an area that uses 10% ethanol, the fuel quality itself is to blame.
If you can buy fuel without ethanol in it, try that. I can't in my state. Also, you might try a fuel stabilizer unless you're filling up the tank on a regular basis.
2011 535i x-drive GT, 2014 i3 BEV