Vertical Thumbnails limit

I have gallery thumbnails set to display vertically but when there are 8 or more then they just get chopped off once they reach the bottom of the main image. There is nothing to select or scroll through them or anything to tell you that there are more to display such as arrows. You only know there are more when you start to scroll through them on the main image.

Have I missed a setting or is this just a limitation of the theme?

Thank you

Theme up to date