The button malfunction in Indesign CC
The button sometimes function in Epub and sometimes it does not work.
If I restart the software, it works. There is a bug.
Alex Ak commented
Try to rearrange buttons on top of everything. You can even create another layer just to place the buttons to ensure they will be on top. If the button contains text or image that cannot be on top, create another empty box to use as a button and bring it to front.
In my experience, when working with Object States, for example, when a small part of one of the states occupies the same position as part of a button below, sometimes the button works and sometimes not, depending on which state is being displayed. I always put buttons on top to avoid issues.