Adding new photos to existing slideshow.

My question is, I started a slideshow project and needed to add some additional photos that were on other devices. I imported those new photos to a new album but when I open up the slideshow to select those new photos, they are not there. They are there in the album on the sidebar. When I started the slideshow project, it automatically opened up the photos Photos to be selected so I am not sure why these new photos are not available to be selected?