My workaround is not to click on the slider, but hover the mouse pointer over it, and it will be activated (highlighted). The I use arrows to move it.
That is not a workaround ... that is how it supposed to work. At least on Mac OS X ...
You do not need to click on the slider to activate it ... if you notice, when the pointer is hovered over a slider, the heading at the left side of the slider becomes highlighted, indicating it is now active. (Likewise for the numerical value of the slider at the right.) This is by design, not by happenstance.
Hover the pointer over the the slider you wish to adjust. Then using the Up and Down arrows on the keyboard will adjust the slider in 5 point increments. Use the Up and Down arrows along with the Shift Key and the slider will move in 20 point increments. If you use the Up or Down arrow keys and the Option Key, the slider will love in one point increments.
Additionally ... if you click on the numerical value at the right you can then manually enter a numerical value for that specific slider.