In case you do what I did:
DO: Rename every one of your artboards with a unique name. [in Sketch]
DON'T: Leave your artboards with the given name. i.e. "Desktop HD" [in Sketch]
DO: Make all of your artboards for one prototype on the same page [in Sketch]
DON'T: Make each artboard for the same prototype on different pages [in Sketch]
It works best to make all of your artboards [individually named] on the same page within Sketch. If you leave your artboard on seperately named pages and if your artboard on every single page has the same name [such as "Desktop HD" or Default] Craft will only "see" one artboard and you will find much needless frustration. It made for a particularly tricky little issue to figure out.