Hello, So I experienced something weird today. On my Sony Experia E5 I was not able to tick on any buttons. I tested this again by making a new project in Unity 5.6.0 and I was able to tap any buttons just fine. Then I proceeded to make a new project in Unity 5.6.2f1 and none of the buttons were tappable. They would stay white... No hover colour change, no tapping colour change. I had no idea where to report this so that's why it's here ~Thanks
This indeed is strange, the Unity bug report page says this "To make bug reporting as quick as possible, we made a bug reporting application for you. When running Unity choose Help->Report a Bug in the menu, or you can access it directly through the executable in the directory where Unity is installed. It will also launch automatically if you experience a crash." The second option might be more where you want to go so you can report this bug. Hope you get everything working soon mate, take care.
I'll make a bug report because I also found it to not work on 5.6.3. But in 2017.2.0b9 it works again.