Lightroom cc "preview not available" upon import

Hello, I have been having difficulty importing from my Canon EOS Rebel T31 (I am using cord to connect camera to my iMac).  I am trying to import 488 photos, 2/3 of them have populated the import screen fine.  1/3 of them say "preview unavailable".  They are .jpg files, not .mov files.  If I import them without the previews, will they be whole files?  Have not tried just clicking on "import" and seeing what happens unless there are previews for all of the images.  Any assistance would be appreciated thanks!