thumbnails are not in uniform rows

I like the plug-in but I’m finding issues with the way the thumbnails are presented. The first row seems fine but subsequent rows are not uniform. Please see here for an example – the first two rows are fine, the second has four thumbnails at the end, the third has two, the fourth has one, etc.

http://www.alechosterman.com/blog/image-portfolio/street-photography/

Any way to control this? Thoughts on why this is happening?

Thanks,
Alec

http://wordpress.org/plugins/portfolio-slideshow/