Trying to make single layer disk image AFTER making dual layer image

Hi all

I wonder if you can help me.

I just made a dual-layer DVD image from a project and it created a 7GB file. Now I'd like to try a single-layer disk image at 4.7GB, something I did with an earlier version of this project, resulting in a watchable disk on a smaller TV.

Having made the 7GB image, Encore won't let me make the 4.7GB version but I don't understand why.

Martyn