The "non-functioning/partially-functioning" ball issue is a hygiene issue

1. Turn the mouse upside down so the ball faces downwards.
2. Spray a -small- amount of WD40 (through the tube they provide) upwards into the space between the ball and the plastic (letting the excess WD40 drip down away from the mouse).
3. Turn the mouse right-side up.
4. Rub a folded clean paper towel over the ball. The movement of the ball will dislodge the shmutz that is blocking the mechanism and the mouse will (ahem) "magically" begin working properly again.

You don't need much WD40. Repeat if needed.

I've done this 50 times in the last few years and it has always worked.

Please don't use WD40 on your mouse. It's oil and oil is slippery and that's the problem in the first place. Oil also attracts more dirt. The only way to really clean the scroll ball is to take it apart and get the oil and hair and lint off the little rollers inside and leave it dry.

This is maybe why the fella who uses WD40 has done it 50 times.

No, do not use WD40. There is a spray solvent that leaves no oily residue, it's perfect for this job. You can buy a spray can of "PCB Cleaner," also called by various names like Circuit Board Degreaser, etc. I bought a spray can at Radio Shack for about $12. It has a little tube so you can squirt it right down the roller ball, roll the ball around, and most of the gunk will be dissolved and cleared by the spray pressure.
I think you should always have a can of Circuit Board Cleaner around for emergencies. It's a solvent that leaves no residue, so it's good for driving out water spills and other liquids. I spilled coffee (with sugar) into my keyboard, usually that's fatal, sugar gunks up everything. I sprayed the keyboard hard with PCB Cleaner to drive out the coffee, problem solved.

I've tried to follow similar instructions in the past, with disastrous results. I'm now using a Logitech mouse, less than half the price. It's a nice mouse, but way too sensitive to dirt.