If the item does not show the image - then this means that the primary image was not selected for that item.
The primary image can be set up in bulk, using the Inventory Import:
Or manually - from Inventory Screen -> Edit Item - "Make Primary" on the bottom of the screen. But if the images are already loaded - in order to bulk change the primary image (thus re-importing the images) you will have to delete the images in order to re-import them using the Primary option. Here is the link on how to delete the images in bulk: