Topic: thumbnail navigation going the wrong way

i made a new gallery page for my website, the same way that i made the others, except the navigation has gone funny.
i have two rows of thumbnails. instead of them going "LTR" across the top row then the bottom row, they are jumping from the top row to the bottom row as they go across.
only when i created the new page has it done this, the old pages made a couple of months ago work correctly.

here is the top bit of the imagedata.xml that i am using

<?xml version="1.0" encoding="UTF-8"?>
<SIMPLEVIEWER_DATA maxImageDimension="400" textColor="0x000000" frameColor="0xcc00cc" bgColor="0xff66ff" frameWidth="3" stagePadding="50" thumbnailColumns="13" thumbnailRows="2" navDirection="LTR" navPosition="top" title= "click on the product code to open the image in a new window" imagePath="" thumbPath="">

what am i doing wrong?

many thanks in advance!  :D