Bose QuietComfort Headphones | Noise cancellation is not working

Make sure you are in "Quiet" mode.

Check your settings in the app to make sure you are in "Quiet" mode not "Aware" mode. In "Aware" mode, external mics on the headset feed outside sounds into the headset and therefore can make other noises or the noise floor seem louder.
See Setting the adjustable Active Noise Cancellation mode.

Do you hear static or distorted audio while in noisy environments?

If the audio is distorted while in loud environments such as construction zones or during take off on an airplane, see Crackling sound heard in noisy environments.

Move away from any hard surfaces.

Reflected sound (caused by using the headset close to a hard surface – like a wall) will negatively affect noise cancellation.

Check for and install any available product updates.

On a computer, visit btu.bose.com and follow the instructions to ensure your product has the latest features and enhancements. See Updating the software or firmware of your product.

Be sure the earcups are tightly seated against the head.

Glasses, earrings and hats may prevent the headphone cushions from forming a tight seal around the ear. Remove any of these items and retest the noise cancelling feature.

Be sure the cushions are securely attached to the earcup.

If the ear cushion is not affixed to the ear cup properly, the noise reduction or sound quality may be affected. For more information, see Replacing the ear cushions.

Be sure the headphone openings are clean and free of debris.

Check for blockages in the headphone mesh grille and headphone opening. Excessive debris can result in low audio, poor sound quality or missing audio channels.