Well, the TLDR of it is the design of the Pros is flawed such that the mesh that rests against the inside of your ear will over time start to get clogged with normal skin oils and etc. There's a microphone under there that adjusts the noise cancelling, and if it's blocked then you'll hear more 'body vibrations' like footsteps, and NC and bass response will be worse.
But you can use "blu tack" (I got Loctite fun-tak) to unclog it and clear the microphone mesh. At least, nearly everyone I've seen comment about it has had success, though I've only had partial improvement. I'm going to get replacements and then keep them maintained. My Apple Store is probably opening soon.
Here's a thread, and I linked to a pic of the mesh in question. People were "sucking" on that mesh, but more people had success with blu tack. Good luck!
Suck on this! (LOL look at the language we've resorted to fix this on our own...)
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