Photoswipe not displaying the viewport properly

I am having problems using this plugin in my website with my bought theme. The responsive Lightbox works perfectly but when I activate the photoswipe it doesn’t display the viewport properly, half of it is hidden behind the header of the page which is positioned to be fixed so I need the viewport to open on top of the entire page so that all the feature are visible. Can you please offer advice. I am also using foo gallery as my gallery for the images!