Problem exporting a master document to PDF: some images appear blank

[nglaude]

I am trying to export a Writer master document (odm) to PDF. In that document, there are 7 other ODT files, each contain images that are linked to image files which are in PDF format (there are a few hundred such images).
(Why PDF, and not PNG? or EPS?, because I have lots of them, and I realized that PDF images seem to be retaining their vector graphic nature when exported to PDF - not true for the other formats)

When I export the master document to PDF, some, but not all images do not appear. It seems to be random as to which ones are kept, but consistent from one time to another.

Oddly enough, when I export the individual ODT files, there are no problems.
Also, if I use the “print to file” function (as opposed to export to PDF), the resulting PDF includes all images, although they have been rasterized as pixel images, as opposed to retaining their vector attributes.

Can anyone help?

This is on Linux Mint 19.3, using the latest 7.0.4.2 version of LibreOffice, and that problem was also there in 6, and also in 7.0.3.

Thanks

[EarnestAl]

The Writer Guide 6.4 has on page 370 " Creating one file from a master document and its
subdocuments", can you use this as an intermediate step? You could break links after updating links in the created file if it still doesn’t export all the images.

[EarnestAl]

If some of the images are stored on a network drive then you’ll need to mount the drive first