It's funny that I didn't realize this was a problem until my son and I got to this story step. I had previously taken a character through solo and had no problem.
When we did this step in a party, my son advanced and I didn't. And of course, I lot the motivation. So I haven't been able to advance. I don't want to use up masteries or gold to try to resolve this issue.
I am hoping that the patch on the 28th fixes this, but until then my son and I can't work through things together.