Cant create new pages with fences but can create with individual icons

Hi

Maybe I am doing something wrong, but I cant create any new pages by dragging fences to the edge and waiting a while. I can however create pages by dragging individual icons. But when I try dragging a fence into the new page it, it just becomes "flattened" against the edge of its existing page. I am using dual monitors with the desktop extended across both monitors.

Any help would be much appreciated.