Missing Onion Skins? And merging scenes.

Hi, noob here. I’ve been drawing a few panels so far and already encountered a problem. In Storyboard 7 Pro, I started with a few panels and had the onion skin turned on. But on the latest panel the onion skin doesn’t work anymore. What do I do? Are the red boxes underneath the layer names useful?

In addition, how do I merge multiple scenes into one? I accidentally made more scenes and not even renaming them will fix this.