A normal Checkbox allows you to click on the text to the right of the Checkbox in order to select the item. It even procs the "Hover" CSS psuedo class when you move the mouse over it, signifying that you can select the Checkbox just by clicking on the text.
The CheckComboBox in ControlsFX requires that you click on the Checkbox itself in order to select the item.
If I didn't explain it well, just drop a Checkbox and a CheckComboBox in Scene Builder, fill the CheckComboBox with dummy data, and try clicking on the text to the right of each to see what I mean.
Take care, everyone!