Since upgrading to 4.3 on my iMac using a trackpad I've found the spot removal tool will stop responding/tracking across the screen. This can happen after a few seconds of application loading or a as long as 30 minutes in. I can only resolve by quitting and re-opening application.
When you select your spot removal tool look at the size of your brush slider on the right side. Chances are it is set to extremely low or even zero. Increase your brish size and the problem will go away. So simple yet it is the fix!