There are only a few things you can try. Depending on the level of trouble you are in they may work or not. Even if you think it might not work, try it anyway, because the only other recourse you have is asking Pottermore to fix it for you and there is a very very long cue in front of you, so it may take a looooooong time.
Go to your browser's cache and empty it. (if you don't know how, google it, or ask here)
Install a fresh version of Adobe Flash.
Close your browser and restart. (and preferably reboot the entire computer)
Go to the fist chapter of the book and test that you can zoom in on the fist moment. ALL moments have 3 layers.
Now go to where the trouble was and try again.
Make sure you only run one browser window with Pottermore in it, from now on.
This is really all you can do, there is no more. If this didn't do the trick, you'll need to contact Pottermore for help.
To contact Pottermore go to the bottom of the Pottermore screen and click 'help', then click 'other', scroll down to 'what's next?' and click 'more help needed', fill out the form.
Please be polite and concise.
They probably already know about this problem and are working on it.
You are not the first or the only person this has happened to.
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