I just tried it again to give a description, and all seems well. But it was frustrating for about a week. I use an iMac with a standard wireless mouse. Normally, if you mouse over the slider button (or even close), the outer circle brightens. If you click the mouse, the center of the button turns white and you can drag it. When it wasn't working, I could click the exact center of the button and it would either flash white or do nothing and I couldn't grab it to drag. Don't know if it was Lightroom, the computer, or the mouse, but it's working now.