I am trying to transfer images that I have taken to an ipad via an sd card and ipad card reader.
The ipad is up to date as far as operating system etc.
I have copied images in a folder to the card.
I have copied individual images to the root directory of the card.
I have copied images to the Nikon created folder on the card.
I have renamed the images to a simple name the same as the Nikon images that are recognised from the owners Nikon camera.
I have used multiple cards.

The ipad will not recognise that there are images on the card ( ipad card reader ) waiting to be transferred.

However, if the ipad owner takes a photo with their camera ( Nikon D5100 ) and inserts a card with that image on it in the card reader the ipad recognises that there is an image there to be transferred.
If I take a photo with my camera and insert a card with that image on it in the card reader the ipad recognises that there is an image there to be transferred.

I can email the image that I have taken and the ipad displays the attached image correctly.

Why can I not transfer images via the card + reader to the ipad.

Am I missing something very basic or is there a known problem?
Google searches so far have not shown any problems.