Hello,
As a rule, images are missing when the process of images upload has been interrupted. This is why you should not leave, close or reload the page while importing, go to the next search page, etc.
To restore images in the imported products, please try to remove a couple of such products (don’t forget to delete them from Trash as well) and re-import them from your Imports History using Re-import button. The import of a product is completed when the progress bar is full and green, active green Import button turns into inactive grey Imported button, and the message telling that the product is imported appears.
Please let us know if it helps.