What EPUB reader are you using? A great deal of EPUB behavior and display depends on the reader, and they vary considerably.
I recommend using a basic, standards-driven reader such as Thorium or Calibre for evaluating EPUB exports. If you need to address other readers, such as Apple's, it is then a job of modifying the export to suit that variant reader's demands.
| Word & InDesign to Kindle & EPUB: a Pro Guide (Amazon)