is because actually you moved the position of the next and previous
buttons
#fancybox-left-ico, #fancybox-right-ico
but not the position of the clickable area
#fancybox-left, #fancybox-right
so you either, move the clickable area to the same position of your
buttons:
height: 30px and margin-top: 100%
or make it bigger to include them
in other words, your buttons now have a margin-top o f 100% so they
are out of the clickable area .... you may want to increase the height
of #fancybox-left, #fancybox-right to 100% + the height of your
buttons (30px) ... you may have to calculate what percentage those
buttons represent