For your reference, there are several potential reasons:
1. This situation occurs if the REDMI Buds 6 is loose during use, preventing the earbuds from switching noise cancellation. Please readjust and wear the earbuds properly before trying to switch again;
2. If the earbuds' battery is low, certain features like noise cancellation will not function properly. Check the battery levels of the earbuds and ensure they are sufficiently charged;
3. If you use touch controls for switching noise cancellation, ensure that you're tapping the correct area of the earbuds.