When I checked these (very few) albums I discovered that my forum members had not uploaded the images individually into the album, but had rather linked to them in the ablum description. This made the album 'empty' in terms of the structure of the gallery - and meant nothing was imported into XFMG, but actually displayed them inline on the album view so they appeared to be okay.
My solution was to create a matching album under my own user account, then edit the album database table with phpMyAdmin to add the correct username and user ID for the original poster. I then gave myself permission to upload to that album and added the images. Once added I edited the media database table to change the username and ID to the original poster, and rebuild the media search.
Just thought this might be useful for anyone who does an import and discovers 'missing' images or albums.