Gallery and lightbox

Hi
I’ve set up a gallery with link settings:
Link to: media file
link image size: large
Link trigger: Lightbox
The gallery thumbnails are set to medium

At the front end, when I click on an image, it loads into a Lightbox ok. Then when I click on the nav arrows, the same image reappears marginally larger. I then click a second time on the arrows and the next image at last appears.

I’m using the Foobox lightbox plugin to provide lightbox facilities.

Is there anything I can do so I don’t have to click twice to get the Lightbox display working? Have I missed a setting somewhere?

thanks
Nick