I have a file in which I merge in a PDF of a page, along with some variable text. I was seeing an ongoing issue where, when turning on Preview, the PDF image wouldn't appear, but there would also be no error.

After trying a few things, I realized that placing the pdf I wanted (on the pasteboard) while choosing an appropriate "crop to" setting, then turning Preview off and then on solved the problem.

I often place images with bleed, and the pdf file in question didn't have a bleed box, so I believe it was failing to import because of improper crop settings.

If that is indeed the problem, I think some warning message would be in order -- merge shouldn't fail because of a setting I used previously while editing a different document.

Using InDesign 16.1 x64 on a fully patched Win10 machine.