Thank you Iliana,
I had copied some of the code from the demo and had
specified on my y-axis so that the actual axis line did not appear. Removing this line from my code caused the plotband to appear. I don't really need to hide the y-axis line, so this is fine. I did not know that the 2 settings would affect each other like that, but hopefully if someone else comes across this, they will have this post to explain it.