At the completion of a quest (Egypt) there's an option to double your reward, when there's an ad this works.
When it tells you to read a story then hit continue how do you do this?
I find that double rewards don't work for reading stories in both diggy adventure and train station with any completed task.
For me, it's very intermittent for reading stories. Something is different about it recently though... if it doesn't show the green Continue, it will sometimes go to the spinning hourglass and then I might get the double rewards. Watching a video works better... but a video doesn't come up very often.