I've heard the rubber has a problem (expanding?) that makes it rub internally. Sounds like you may need warranty service. Hopefully Logitech won't object, especially if it gets worse.
My VX may suffer from the same problem (if I keep it). We'll see!
Mine seems to be alright (I own both a MX Revolution and a VX Nano)
Just call Logitech, they'll send ya a new one for free, their customer service is rediculously good!! (I got this MX Revolution inplace of a dodgy MX-1000!!!)