Only new images showing in gallery

Hi,

I have previously created a gallery of 30 images which looks fine. I went in today and added two new images from the media library to the gallery making 32 images. When I update, only the two newly added images show, the original 30 have vanished.
If I go back in to the gallery and remove the two newly added images from the gallery and update, the original 30 images are visible again. What I am doing wrong?