I'm sure mighty mouse owners have, at some point, had this problem where the ball won't scroll down but have no problem scrolling up or sideways. Naturally, I Googled when it happened to me. It started to limp a while ago, but it just locked down today. Many, as I've found on various forums, initially thought this might be an application problem and they'd restart their computer or sell the mouse on eBay. The problem, however, is very simple. Gunk. It will build up after a while and faster if your hands sweat. Guitar players have the same problem with strings and some need to change strings every few days. Rage Against The Machine's ex-guitarist Tom Morello advised in an interview, "Always wash your hands before you play. Your strings will last longer." Good advice.
Okay...when it gets to the point where your ball won't scroll down anymore, it's time to unplug the fucker from your keyboard. Lay a clean lint-free cloth on the table (I used soft T-Shirt), dampen a portion of it with water (alcohol and other stuff not required at all - just water), turn the mouse upside down, press hard (it's okay) and scrub (long rolls) on the damp part like hell. After a few seconds, move to a dry portion and repeat. Repeat it back and forth between the damp and dry parts. Don't rub gently; press the fucker down hard and rub vigorously...like when you're...never mind. If it's a white T-Shirt, you'll notice some dirt being smudged on it. A couple of minutes of intense rubbing should dislodge most, if not all, gunk.
I'd thought mine needed a major surgery, you know, like cracking the bottom open and all. Don't try it because it might ruin something else inside. Many have tried opening it up - some succeeded - after being frustrated with the Apple method. I tried the Apple method first. It didn't do shit. It looked like the dude was wiping his eyebrows ever so gently. It wouldn't hurt to press the ball down and use the air-blower thingy between the cracks. I just blew on it hard with my lungs. Just don't spit in it...unless you're into that lifestyle.
Guess how I tested if my scroll ball works beautifully. I wrote a long-ass post about cleaning the scroll ball and am now (right now) scrolling the fuck up from...weeeeeee.....the top.....to...weeeeeeeeeee....right back here. Sweet. Scrolling as smooth as a new born mouse. Don't trust your fingers to know if the scroll has gotten its health back? Then listen...and you'll hear the sweet sounds of smooth clicking - like someone winding a mini bicycle chain. Ahhhh!!