Can't insert full size images

I realise this problem might sound like something from years ago but..

Latest WordPress, child theme + Twenty Twelve in use. The child theme is really simple. amandathomas.co.nz is the site.

My client presizes her images before uploading. Landscape ones are 805×537, portrait ones are 400×600.

I edit a post, add media, go to the Library, select a bunch of images (a mix of landscape and portrait), make sure ‘full size’ is selected, insert them, save draft, view the post.

Not all of them are full size! Apart from the first landscape image, which IS full size, the other landscape images are inserted as 625×417.

I am tearing my hair out. Is this something to do with the new responsive image parts of 4.4 onwards?