Could it be a browser issue?
I tried playing it in different browsers and the same thing you describe happened once - after ch1, there was the 'play again' screen. When I refreshed it, it let me play up to the end of ch2, then the same thing happened. Refreshing it again, it finally let me play the whole thing.
This makes me believe that it's not code-related, because it worked fine in all the other cases. So the only thing I can think of is some cache/cookie issue?