Using an iMac on Catalina.
When I import images on an SD Card from my camera into Photos, it never imports all the images in one go.
For instance, if I want to import 30 images it will import maybe 27, then I’ll have to go back and import the remaining 3 in a second move.
This applies to two different SD Cards from two different cameras.
Your second suggestion works !
By default the images were imported to 'Library'......... and the fault appears.
But if I import to any other folder the problem isn't there; it imports all the images in one move. I just have to import to any folder other than 'Library'.
Adding an update......... I thought I had solved the problem - but I was wrong.
Importing into another folder still brings up the same fault; there are always one or two images not imported during the first import. Even with a new folder, I still have to complete the import in two moves.
Two things to try.
1 - Reformat the memory card in camera. This has fixed similar issues for others.
2 - Copy the files first to a folder on your system drive, and then import from there.
Importing images into Photos
