"Fixed Layout ePub" is not producing fixed layouts

I've used InDesign CC several times to create fixed-layout ePub comic books that worked well on all the popular platforms. Those files were built using a Mac, for whatever that's worth. After switching to Windows, I tried the same procedure I've done before and the ePub exports like a reflowable comic book -- awfully! The dialogue balloons (created in InDesign), sound effects (largely copied over from an Illustrator workspace) are treated as separate graphic elements that sometimes show up on their own blank pages. Text is displayed on separate pages, underneath the graphics that might be there.

It's a mess. This should be simple. I've done it a million times before. What am I doing wrong?