Cannot import .tif files into LRCC

Hello,  I am trying to import .tif files that were scanned slides into LRCC.  The first folder of 20 imported as per usual, no problem.  Every subsequent set of images I try to import will not work.  I click "import", navigate to the folder and where there should be preview images, it shows grey block w "preview unavailable for this file", the boxes are checked.  I have "add" selected.  When I hit import, a screen pops up with "some import operations were not performed" and An unknown error occurred and (12) which is the number of photos I'm trying to import... any help???? I tried drag and drop, same result, I tried "move" instead of "add" same result.  Don't understand why first set of 20 went over fine and now nothing.  I have hundreds of these files to move into LR.  Thanks