Your import software is likely set to copy only new files. The simplest solution is to bypass the automatic importer and copy the photos manually like regular files.

Connect the memory card or camera so it appears as a drive on your computer, open it in File Explorer/My Computer, find the photo files, then select the images you want and copy them to a folder on your hard drive.

If you prefer to keep using the import program, one workaround is to remove the previously imported copies from the destination folder first, then run the import again so the software no longer thinks those files were already transferred.

Manual file copying is usually the most reliable way to re-transfer everything from the card.