Ever looked at your scrollwheel? If you've had if for a few months then it's probably filthy.
Ordinary-looking mouse - a bit mucky, but normal enough
Let's zoom in on that wheel. OMG. What is that crap?
Scrape it all out with a spikey screwdriver thing.
Dirt, sebum, bits of skin. Yep, this is human snail trail.
Oooh, and we can roll it up into a tiny ball! Woo hoo!
Ah, all clean now. Sort of.