If it's a 21st century car there's an adjustment than can often cure this. Open the cap.inside is a moveable alluminium ring with a spring loaded flap. The rotated position ofnthis is critical.
It moves easily with finger pressure. Try positioning the hinge of the flap at 6 o'clock position. You may find this an instant fix.if not, try it at 12. Sounds daft but has worked for many of us.
Nick
