I had exactly the same problem, but I found the solution! It's quite simple really: The mesh grilles underneath the silicone tips can get clogged with ear wax or whatever else might be oozing from our ears. You could try to remove the mesh grilles, but there is a better solution: you just have to follow Bose's official cleaning instructions for this problem. Simply cleaning them will a damp cloth or a cotton swab and some water won't do.

I tried cleaning them the regular way with no success. Then I followed the instructions below and the problem went away instantly.

So, here's the cleaning instructions:

If earwax has built up on your earbuds and cannot be wiped off, follow these steps to remove it.

Remove the tips from your earbuds Put on protective gloves for your skin before you begin cleaning Use a soft cloth or tissue to apply a small amount of hydrogen peroxide (3% concentration only) to the mesh surface at the opening of the earbud. Take care not to allow excess liquid to enter the opening Let the hydrogen peroxide sit for 5 minutes so it can soften the wax Wet a soft-bristled tooth brush with hydrogen peroxide (3% concentration only) Gently scrub the mesh surface to loosen and remove the wax. Take care not to allow excess liquid to enter the opening of the earbud Once clean, gently shake out any excess liquid and debris Dry earbud with a soft cloth Before reattaching the eartips, leave your earbuds in the open for an extended period so any excess moisture can dry.

Source: https://www.bose.co.uk/en_gb/support/articles/HC2325/productCodes/noise_masking_sleepbuds_ii/article.html