Photos won't let me place photos in a book. They keep snapping back.

Photos is driving me crazy. I'm creating a book from our Africa photos, and when I try to drag a photo into the photo frame on a page layout the photo keeps snapping out, even though it ends up in the "placed" photos. I've made many photo books using iPhoto and never once had a problem, but now I'm ready to move to Shutterfly or another of the online photo book sites.

Has anyone else had this problem? If so, how did you solve it??