Previous/Next Custom Buttons Inaccessible

I use Storyline360. I created custom previous and next buttons to go with the subject matter. They are in the lower right of each slide. They have worked fine throughout development, but as I was doing a final checkout before sending to the client, the custom previous/next boxes are behaving abnormally. The boxes should change shade when learners hover or click anywhere over them. Now they act as if all but the very top of the buttons are covered by a hidden box.  If I move them up enough, they become accessible. The START button on slide 1 exhibits the same properties. I can move them up, but that will interfere with slide content on some slides. Is this a Storyline setting or system setting issue, or something else?