Ever since the main breeding page was updated to the mobile format I've been intermittently experiencing an issue where the stallion drop down is unresponsive when I go to use a mare for brood. As far as I can tell it happens whenever the mare's color description exceeds one line. This is the girl it happened with today.

I've had this issue as well.

The revamped Breed page will be put in on Tuesday. See if you still have that problem after it's updated.