This is not advice, but simply my experience.
I've had Bose, Ozlos, replacements of each, A20s - three pairs. I've frequently had one bud go randomly quiet, I've cleaned carefully with alcohol, sticky tape and blu tack/putty. Some solutions have helped one day but not the next, once I fixed the left bud and the right stopped working... I feel like I've gone through so many of the experiences being shared here.
Today I did something terrible and now my latest dud earbud - my Ozlo - WORKS. I accidentally ripped the little black mesh 80% off. (Fare thee well, warranty.)
This gave me access to the next bit - an inset bit of metal within the plastic case. I sprayed alcohol on a mini flossing brush, and gently jiggled it around on top of the metal and just into the little gaps between the metal and the plastic.
More ear wax came out. MORE. GROSS. Most importantly, volume restored!
Everyone carefully cleaning the mesh - please dont do what I did because it was so risky and dumb! But. I wanted you all to know that there may be more ear wax lodged in a place that cleaning the mesh will never reach.