[Plugin: NextGEN Gallery] Thumbnail problems after image reorder

Does anyone know how to stop gaps appearing in the thumbnails view after reordering the images in a gallery?

I’ve tried all the option settings. The one that reorders as I want them is ‘custom’. But this puts gaps between the thumbnails.

See https://goyt-valley.org.uk/photos/friends-family/.

Any advice would be much appreciated.

Thanks