In PM6 (and PM+) I'm noticing that the Loupe feature works for some files, but not others. (I get a spinning gear icon that lasts forever.) Unfortunately I can't find a common denominator. It works on some DSLR NEF files but not other NEFs from the same camera. It works on some scanned negative TIFF files but not other TIFFs from the same scanning source. But it's always the same files that do (or do not) work.
PM6 release 6R3558 (but it happened on the previous release, too). Mac OS 10.13.6.
Any ideas?