This issue actually has more to do with the browser’s changing the way they work with iFrames for security reasons. The reason why it is only happening in JW 7 is that is the only version we actively maintaining.
The issue is caused by browsers requiring all iFrames to include the allowfullscreen parameter if you want the content to be allowed to go into full screen. The shadowbox.js script that you have on your page is creating the <iframe> code without this parameter and would need to be updated in order for this to work.
Also, please note that browsers, especially Chrome, will stop allowing any insecure content from going fullscreen in future updates to their browser. The URL you are passing into the playVideo() function is http://content.jwplatform.com/players/zxnv699E-JZSbrDCz.html. All you would have to do to future-proof this would be change it to https.
Please let me know if you need any more help or have any other questions.
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