Bose QuietComfort Earbuds II noise cancellation or transparency is not working correctly

The checks in more detail

Try each tip size and each stability band, since the earbuds come with several, and expect that your two ears may want different sizes. A tip that is slightly small gives a thin sound and weak low-frequency cancelling, the sign that the seal is poor. Seat the bud with a twist, hold the ear gently and run the test again. Check the modes. Quiet gives full cancelling, and Aware lets outside sound in through the microphones, which is deliberate. A custom Immersion level can sit between. Make sure Quiet is selected and set high.

Wind is a known limit: in a breeze, the microphones hear turbulence and the system reduces its work to avoid a rushing noise, so shelter helps. Very sudden sounds, such as a door slam, are not cancelled because they are too fast for the system. Clean the small microphone holes on the outside of each bud with a soft dry brush, because dust stops them hearing the noise they are meant to cancel. Check that one bud is not dirtier than the other.

Update the firmware, since Bose tunes cancelling in releases. If cancelling stops suddenly on both buds, put them in the case for a minute and try again, and reset the earbuds if it persists, using the app or Bose's help pages for the steps for this model. If cancelling is poor even with a perfect fit and Quiet mode, contact Bose with the serial number. Remember that strong cancelling reduces your awareness of traffic, so use Aware mode outdoors.