Images do not show in PDF/X

I created a PDF from a Word document; Word uses PDF/A as the output.  The file looked fine on screen and was printed correctly as a paperback using a print on demand service.

The author I am working with chose a different POD company for the hardback.  This company wants PDF/X, so I used Acrobat DC to convert the PDF/A version.  When printed, five of the images dropped out, with correctly sized blank space left on the pages; all other images priinted correctly.  The images in question are missing on screen when the PDF/X is viewed in Adobe Reader, but other PDF viewers (Sumatra and PDF-XChange) show them as expected, which is very strange.

I am guessing that there is some difference in how PDF/X handles images, but do not know.  All the missing images are grayscale and of small size.  I'm willing to edit the images if needed but don't know what to do, and so would appreciate any suggestions.  I am on Windows 10 with most recent versions of all apps.