Why might Frame graphics <sometimes> not appear in PDF?

I've got a multi-chapter FrameMaker document with screenshots.  The screenshots are JPG or GIF files imported into anchored frames.  They appear in FrameMaker and all properties seem to be identical.  However, in the generated PDF, some of the screenshots are blank white spaces (when clicked, they are a blue box), and some of the screenshots display as they should.

I've compared the ones that don't display with the ones that do and cannot figure out the difference.

What could I be missing???