Unfortunately, there is currently a known bug whereby elements which have explicitly been assigned a CSS 'position' are not covered by the fullscreen gallery (they should be). This bug has already been addressed and will be fixed in the next version of Juicebox.
In the meantime, there are a couple of workarounds:
(1) Set useFullscreenExpand="TRUE".
... or:
(2) Set expandInNewPage="TRUE". When the gallery is expanded, it will be displayed on a page of its own rather than on top of the embedding page. Please note that this option is available in Juicebox-Pro only (and not in Juicebox-Lite).