I don’t think it was mentioned outright in Thoughts From Iceland, but on my first trip, it was too cloudy to see the aurora the night we were supposed to try. It’s a pretty common situation for tourists in Iceland, actually.
On my second trip, it was too cloudy again at first. But we took a break to go get ice cream (which was delicious), and then went back out looking again. Well, the second time around, we managed to find some!
It was very faint, and the photos I took looked a lot better than it did with the naked eye. But it was still very cool to experience, and very satisfying. : )