Thanks for the responses. I neglected to save the version with the problem, but I think I might have had a tiny corner "nipped" off the lower, front corner that was stopping the face from moving back. Can't know for sure due to my error in saving.

New to SketchUp and just found this forum. I have what seems like a newbie question. As shown in the attached image, I've drawn the profile of a set of stairs but I cannot push the highlighted area back to remove that area. I can pull it out, just not push it in. I've read something about thin edges that cut and thick edges that sit on top of a surface. I don't know how one determines which is created or if it's possible to change an edge from a "thick" edge to a "thin" edge.