Ex- and Import media files

Hi there,
I'm experimenting and learning a bit about exporting sites form a MS network to a WP standalone install. I have done this before, and then the importing of the media files in the library went smooth.
Not this time. They are NOT imported. Only a plugin like 'Add from server' works, but it's a lot of clicking work...
I wonder: what can be an explanation?
Can it have to do with the webhost or server settings?

Just wondering and hoping to learn.

Thanks for thinking with me!