ZC-1.5.5e, PHP 5.6.32, Database patch level 1.5.5 , IH 5, ZLB 1.6.5

The Next button does not appear on the main image.
The additional images have the appropriate next and prev buttons.
ie. The 1st Additional image has the next button. The 2nd additional image has the next and prev buttons.
The last additional image has the prev button.
Everything else is working fine.

any help would be appreciated