What version of Tableau are you using? I wouldn't imagine that the version would be an issue, but it's worth asking anyway.
I've just opened your example and I'm able to see the labels:
If you haven't already, save your file and re-open it to see if the labels appear.
Let us know how it goes.
I am using v10.0.1. Interesting, I guess I will have to open a ticket with Tableau to resolve this issue.