If your flipbook can't be opened in fullscreen where you embedded it, here are some ways to fix that:
Check if the allowfullscreen="true" parameter is present in the code;
<iframe src="https://cdn.flipsnack.com/widget/v2/widget.html?hash=fhpiofaak&bgcolor=EEEEEE&t=1535714119" width="640" height="385" seamless="seamless" scrolling="no" frameBorder="0" allowFullScreen="true"></iframe>
Make sure the Flipsnack iFrame was not inserted in another iFrame embed code. If that is the case, the parent iFrame must have the allowfullscreen="true" parameter as well.
Please note that the flipbook will not open in fullscreen on Wix because unfortunately, the platform doesn't have support for the allowfullscreen="true" parameter.
If you want to embed your flipbook on Wix, please read this article.