In Dynamics CRM 2013 or 2013 SP1, sometimes you will find that an option set won’t drop-down when you click on it. It doesn’t happen every time, but occasionally, when you click on the option set with your mouse, the option set will not drop down, but you can toggle through the options with your keyboard arrow keys.
This is a known issue. If you find that this is happening with one of your option sets, one workaround is to position the label for the option set to the left of the field. After changing the label position, the option set should drop down as desired.
Thanks to Scott Jung for this tip!