Same thing happened to mine after about 50 hours of use. Amazon were not interested and as of today, I have had no response from Sennheiser (still under warranty). I suggest you remove the rubber shroud from the buds and check for accumulations of ear wax (yugh I know). Remove any deposits. Then (I suspect not recommended by the manufacturer) place the buds over the spout of a boiling kettle (plugs facing up) to soften any deposits inside the plugs and dry thoroughly. Hey presto - they are repaired!
As these plugs go well into the ear canal, ear wax will impede the performance (no matter how often you clean your lugs out). This might not work for you but if your plugs are out of warranty and are now rendered useless, give it a try as you have nothing to lose.